I'm using this thread to record some possible issues I've noticed in various Tolkien books.

Sir Gawain and the Green Knight, Pearl and Sir Orfeo

Sir Gawain and the Green Knight line 1610 "with bred blent þerwith his braches rewardez" is translated as "and with them [boar's entrails], blended with blood, the bloodhounds rewarded", which "bred" is translated as "blood". All other translations of this poem I've found translated it as "bread". In both Tolkien & Gordon's 1925 work and the manuscript in the British Library, line 1610 reads "bred". And the glossary in Tolkien's 1925 book says bred=bread, which can also be confirmed with any Middle English dictionary. "bred" also appears in line 891 and 1361, both translated to "bread" in Tolkien's translation.

Annotated Hobbit 2nd ed.

Now with the publication of so many new materials, there're of course some outdated statements in this book, like the start year of The Hobbit, Tolkien's work in "Abingdon Chronicle", Tolkien's confusion of Thror & Thrain. But we'd only focus on possible "mistakes".

The runes on the title page read "published by the Houghton Mifflin Company". HarperCollins's UK version copied this rune without knowing its meaning Decades ago Houghton Mifflin published The Hobbit with runic "published by Allen & Unwin" on its cover, now they finally revenged to the successor of A&U.

P171 Den Islanske Bolig i Fristatstiden
The title is actually "Den Islandske Bolig i Fristatstiden", see https://books.google.com/books?id=MWMDAAAAYAAJ&pg=PA251

P308 "the blood hounds rewarded."
In the quoted source, this sentence is "the bloodhounds rewarded."

P377 for "pp. 28-89" read "pp. 288-289".

Bibliography - Chinese
[The Hobbit]. Jinan: Tomorrow Publishing House, 2000. Translated by Xin Piao; illustrated anonymously.
[The Hobbit]. Nanjing: Yilin Press, 2001. Translated by Li Gi.
The Pinyin name of the translators are wrong. They should be Xu Pu and Li Yao.
More details about Chinese translations can be found here: https://lawrencemvc.wordpress.com/

The History of the Hobbit 2nd ed.

III-iv: one particular phase of the moon would only coincide with a specific night of the year roughly once per century.
In LOTR appendix we find their calendar year length is 365.242 day. And their lunar month length should be the same as on earth: 29.5306 day. 365.242*19/29.5306=234.997. Therefore the minimum cycle period is 19 years, the so-called Metonic Cycle.

XIV.i: the missing ligatures can be restored with confidence: [picture] A literal transcription, with vowels indicated by diacritical marks in the original enclosed in parentheses and restored letters obscured by the ladder given in italics, reads as follows: G(O)LDTHR(O) R THRA(I)N / AK(E)RST B (E) THE TH (E) F / TH [ror] TH [rain]
The Tengwar on the Jar is not a tehtar mode with vowels indicated by diacritical marks, but a full mode with vowels indicated by standalone letters. It also uses phonemic spelling, where diacritical marks are added to vowel letters to represent the modification of vowel sounds, like [əː], [ai], [əʊ], [i:]. The details of this mode can be found in Addendum.vi. With Tolkien's instructions we can easily read the words:
gəʊld θrɔ:r θrain / akəːst biː ð θiːf , i.e. Gold Thrór Thrain / accursed be the thief
The restored picture also mistakenly took the tengwar for [əʊ] and [ɔ:] as tehtar for "o".

Addendum vi P909 [a tengwar word]=dance; but =hats;
The first Tengwar word is actually "chance" rather than "dance".

Addendum vi P910 [a tengwar sentence]=Tom, the piper[TN8]
For "piper" read "piper." The tengwar sentence has a full stop sign at the end.

Addendum vi P910 pronunciation varies between [ā] and [æ] as in [a tengwar word][added above the tengwar in neat ink: glass]
This one is Tolkien's error, the tengwar word he wrote is actually "class". The "a" in both class and glass can vary between [ā] and [æ].

Lord of the Rings: A Reader's Companion, 3rd ed.

p. 523 last paragraph, it says:
770 (III: 43). Forlong the Fat, the Lord of Lossarnach – In Appendix F it is said that Forlong was a name ‘of forgotten origin, and descended doubtless from days before the ships of the Numenóreans sailed the Sea’ (p. 1129, III: 407).
However, in both copies of the 50th anniversary edition that I checked, it says “from the days” instead of “from days”. It seems to be a change made in the 1994 edition.

p. 548, ll. 15-16: For “Book VI, Chapter 7” read “Book V, Chapter 7”.

p. 633, last paragraph. This paragraph seems to be misplaced. It should be the note for later text.

The J.R.R. Tolkien Companion and Guide 2nd ed.

Chronology 21 December 1927 Tolkien, as ‘Father Christmas’, writes a letter dated 21 December to the entire Tolkien family as well as Aslaug (one of the Icelandic au pair girls)
In _Father Christmas Letters_, her name is actually "Áslaug". The name can also be confirmed in an interview of another Icelandic au pair girl Arndís Þorbjarnardóttir (https://timarit.is/page/1928715#page/n25/mode/2up)

Chronology 28 October 1971
There are two separate entries for this date, which should be combined. Or was one entry misdated?

Chronology Notes: ?1926–?1930 One of the Icelandic au pair girls, Sigriður Þórarinsson, was introduced to the Tolkiens by May Morris, daughter of William Morris (with whom Sigriður’s cousin Eiríkr Magnússon translated saga literature). The story of her stay with the Tolkiens in Oxford, and of encounters other members of her family had with Tolkien, is told by B.S. Benedikz in ‘Some Family Connections with J.R.R. Tolkien’, Amon Hen 209 (January 2008).
According to the Amon Hen article, Eiríkr Magnússon should be Eiríkur Magnússon (he also has a wikipedia entry), and Sigriður Þórarinsson should be simply Sigriður, or Sigriður Benedikz/Sigriður Benediktdóttir. Traditional Icelandic name usually doesn't have a surname, and their "surname" is the father's name followed by -son ("son") or -dóttir ("daughter"). No girl would have Þórarinsson as "surname". According to the Amon Hen article, the three generations in his family are named: Benedikt Sigurður Þórarinsson, Eiríkur Benedikz and Sigriður, B.S.Benedikz. Apparently, the 2nd generation used the father's name Benedikt as a real surname. Therefore Sigriður's full name is probably Sigriður Benedikz (new style) or Sigriður Benediktdóttir (old fashion). But simply calling her Sigriður is enough, as most Icelanders would do.
Sidenote: Beside Áslaug and Sigriður, we know two more Icelandic au pair girls: Arndís Þorbjarnardóttir and Rúna, from an interview in Feb 28,1999 in the Icelandic newspaper _Morgunblaðið_. https://timarit.is/page/1928715#page/n25/mode/2up Transcription and partial translation at https://svanurg.blog.is/blog/svanurg/entry/1065767/

Reader's Guide Winter’s Tales for Children 1. "verses Tolkien had written for an American fan, Rosalind Ramage."
It seems Rosalind Ramage then lived in Wells, Somerset, UK. See _Tolkien: Maker of Middle-earth_, 96.

Tolkien: Maker of Middle-earth
P5 For consistency, "ESSAYS" should be added to the Table of Contents, below "Foreword", in the same font as "CATALOGUE".

P48. In the diagram Kurdish came from Middle Persian. However, most modern scholars consider Kurdish parallels Persian. They are in the Northwestern and Southwestern branches of the Iranian languages,respectively.

P96. ‘Down she came without damage: / Rosalind Ramage.’
For "." read "!", as can be seen in the picture.

P118. Gift card from ‘Ronald’ to his father
For ‘Ronald’ read ‘Wanild’, as can be seen in the picture. I guess that's the very purpose of the quotation mark.

P157. May God bless you my dear John Ronald
There is a comma after "you" in the picture.

P207, 339. For consistency, "(shown overleaf)" should be added at the end of the item description.

P212. "three designs representing the towns of Tavrobel" should not be in bold.

P216. For "The stories were the thing" read "The mere stories were the thing", as the text in Letters says.

P227. For "Tolkien 1986, p. 303." read "Tolkien 1977, p. 252." The quoted text is actually from The Silmarillion, not The Shaping of Middle-earth. (SoMe has similar sentences, but not exactly the same)

P284. "King Solomon’s Mines" and "Morte d’Arthur" should be in italic.

P290. For "two other pages" read "two other sheets", for "This stray page" read "This stray sheet".
According to History of the Hobbit, the earliest manuscript has a "fragment of six pages (three sheets)" in total. So the three "pages" mentioned here are actually all the surviving "six pages" of the earliest manuscript.

P378. A working map had been in existence for over ten years (see pp. 380–1).
For "380-1" read "398-9".

P394. For "100 square miles" read "100 miles" (or "10000 square miles").

Also, I have a little doubt about note 2 on P106. The contexts seem to suggest the two quotes came from TWO different letters from Tolkien, one in reply to Margrethe's 24/10/1970 letter, another in reply to her 1971 Christmas card. However, the notes say both replies are in the same Tolkien's letter drafted in Jan 1971, which also implies Margrethe's Christmas card was for 1970 Christmas, but Tolkien received it only in Jan 1971. I think this scenario is a little weird, and wonder if there is any typo in note 2, like mistook "1972" for "1971".

The Worlds of J.R.R. Tolkien
P.54 they are gradually sailing away from havens in Lyonesse
It seems Tolkien never used this spelling directly, he always used ‘Lionesse’ (Lost Tales II 313, 334; Lays of Beleriand 140; Lost Road 85).

P.66 [27]
[27*] seems to fit here better, since there are comments in that end note.

P.69 Tolkien’s notes show that he intended the name Dorwinion to mean ‘Land of Youth’ in Elvish. On the other hand, the region’s wine exports, put together with the common element dor ‘land’, inevitably suggest another meaning – ‘Wineland’.
In Tolkien's notes on the map for Pauline Baynes, he first wrote something looked like ‘MILDOR [Wine-land]’, then changed it to ‘Dorwinion’. https://www.tolkiendil.com/essais/geographie/carte_tolkien_baynes I think this suggests Dorwinion probably doesn't mean Wineland.

P.88 Eiger and Monch
The correct spelling seems to be Mönch, which is used on the map and its legend in P.86.

P.137 a three-dimensional model of a medieval Norse hall reconstructed for an 1892 exhibition in Copenhagen.
As Annotated Hobbit and the source "Den Islandske Bolig i Fristatstiden" (https://books.google.com/books?id=MWMDAAAAYAAJ&pg=PA274) show, the exhibition is in Spain. https://www.jstor.org/stable/658309 I guess the sentence is better rephrased as "reconstructed in Copenhagen for an 1892 exhibition".

P.177 Tolkien’s walk to school would have taken him across the Birmingham Canal were it opened into Gas Street Basin, its wharves ahum with business.
I think "were" is a typo of "where", as one can verify in the map.

P.190 2 Andrew Higgins, unpublished PhD thesis
This thesis ‘The genesis of J.R.R. Tolkein's mythology’ is published and available online since 2016: https://repository.cardiffmet.ac.uk/handle/10369/7528

Since Haloisi Velikë is in the index, I think other names in I Vene Kemen in p.68 should also be indexed by adding italic "68" in corresponding entries: Magic Isles (I Tolli Kuruvar), Taniquetil, Tol Eressëa, Twilit Isles (Tolli Kimpelear). Similarly, the names in the heraldic drawing in p.48 should be indexed by adding italic "48" in their entries: Celbaros, Kortirion(Cortirion), Ecthelin, Tower of Ingil, Meril(Miril), Ranon.

P.207 Tolkien, JRR - Poetry:Death of St Brendan 34, 192...Imram 34, 192;
better add "80-81", where the poem is quoted.

P.207 Trees. 35, 36, 39, 71, 110, 113–31, 154, 160, 160
duplicated "160". P.160 has no picture. "Tree" is not mentioned in either texts or pictures in neighboring pages.

April 2023 update: The index has "1931 visit 21, 71, 74" under "Cornwall" entry. p21 says the Tolkiens spent a holiday in Lamorna Cove, Cornwall in 1931; p71 describes a 1928 visit to Lyme Regis, Dorset; p74 says the Tolkiens spent a holiday in Lamorna Cove, Cornwall in 1932. Letters (notes to nos 76, 257) and Chronology both say the holiday was in 1932. It seems p21 should be "1932", and the index should be "1932 visit 21, 74".