As the "second-generation" proof of the Classification of Finite Simple Groups is being written up in the volumes by Gorenstein, Lyons, Aschbacher, Smith, Solomon, and others (see e.g. [this question](https://mathoverflow.net/questions/114943/where-are-the-second-and-third-generation-proofs-of-the-classification-of-fin)) certainly much of the work will go into fixing minor (and possibly major) issues and gaps in the proof, since the first announcement in 1983.
Here are two such gaps:
1. The classification of quasithin groups. G. Mason claimed a proof in an unpublished manuscript in 1981, but this was found to contain serious gaps. It would not be until 2004 that this gap would be fixed (see [this behemoth](http://homepages.math.uic.edu/~smiths/papers/quasithin/quasithin.pdf), [1,2]).
2. In 2008, Harada and Solomon [3] filled a minor gap in the classification by describing groups with a standard component that is a cover of the Mathieu group $M_{22}$, a case that was accidentally omitted from the proof of the classification due to an error in the calculation of the Schur multiplier of $M_{22}$ (from the [Wikipedia page for CFSG](https://en.wikipedia.org/wiki/Classification_of_finite_simple_groups)).
I would like to see a longer such list! Thus:
**What other (major or minor) gaps have been discovered, and subsequently fixed, in the proof of the CFSG, since the announcement in 1983?**
Of course, if there are any gaps that are "known, but with a known fix" (but which have not yet made it into the aforementioned second-generation proof), then these would also be interesting to know.
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References:
[1] <cite authors="Aschbacher, Michael; Smith, Stephen D.">_Aschbacher, Michael; Smith, Stephen D._, The classification of quasithin groups. I: Structure of strongly quasithin $\mathcal K$-groups., Mathematical Surveys and Monographs 111. Providence, RI: American Mathematical Society (AMS) (ISBN 0-8218-3410-X/hbk). xiv, 477 p. (2004). [ZBL1065.20023](https://zbmath.org/?q=an:1065.20023).</cite>]
[2] <cite authors="Aschbacher, Michael; Smith, Stephen D.">_Aschbacher, Michael; Smith, Stephen D._, The classification of quasithin groups. II: Main theorems: the classification of simple QTKE-groups., Mathematical Surveys and Monographs 112. Providence, RI: American Mathematical Society (AMS) (ISBN 0-8218-3411-8/hbk). xii, pp. 479-1221. (2004). [ZBL1065.20024](https://zbmath.org/?q=an:1065.20024).</cite>
[3] <cite authors="Harada, Koichiro; Solomon, Ronald">_Harada, Koichiro; Solomon, Ronald_, [**Finite groups having a standard component \(L\) of type $\widehat M_{12}$ or $\widehat M_{22}$.**](http://dx.doi.org/10.1016/j.jalgebra.2006.09.034), J. Algebra 319, No. 2, 621-628 (2008). [ZBL1135.20009](https://zbmath.org/?q=an:1135.20009).</cite>