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. 

One major gap in the proof of the CFSG that has been fixed since the announcement in 1983 concerned 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 G. Mason claimed a proof in an unpublished manuscript in 1981, but this was found to contain serious gaps.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)). 

For another gap, on the [Wikipedia page for CFSG](https://en.wikipedia.org/wiki/Classification_of_finite_simple_groups) one finds that 

>"[in 2008], Harada and Solomon fill 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 $M_{22}$."

**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.