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!