Okay, here is the problem in full:
1. Possess your opponent 3 times, Chapel away his stuff and buy 2 Coppers
1b. Opponent buys a Copper
2. Do the same thing, but buy a Pawn
2b. Opponent buys another Copper and has 5 cards already
3. KC the Pawn, Chapel the Coppers, buy a Great Hall, bring yourself down to KC-Poss-Ch-Pawn-Great Hall
3b. Opponent buys his 6th card, 3rd Copper
4. Whatever you do, there is a chance your opponent has drawn KC-Poss-Copp-Copp-Copp (no Chapel) so you can't Chapel his stuff anymore, leaving you open for a KC-Poss yourself at which point your opponent can grab the lead
And if your opponent has Chapel-Poss-KC-Copp-Copp you can't use that turn to buy a Great Hall