TITLE: Finite Simple Groups gap recently filled
QUESTION [7 upvotes]: The recent paper,
"Aftermath," by
Peter Cameron (arXiv:1111.4050v1), contains this remark concerning
the classification of the finite simple groups:

The Classification of Finite Simple Groups [16] is the greatest collaborative effort ever in mathematics, running to about 15000 journal pages. (Ironically, although the theorem was announced in 1980, the proof contained a gap which has only just been filled.)

Can someone explain (at a high level) what was the gap and who filled it?
(Cameron gives no reference.)  Thanks!

REPLY [11 votes]: Answering as a CW so the question will not appear as unanswered.
I believe it may be a reference to the classification of quasithin groups; this was filled by Aschbacher and Smith in a series of papers around 2004. 
The fact that this was a gap that needed to be filled only became apparent as the proof was being organized for re-publication (in the AMS book series).