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  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!