Was "unbreak now", but we've rolled back the bad change.
It looks like GC donors are being sent to the error page after a good transaction. See order_id 8274273886 and Civi IDs 12107504 and 12107844.
Nov 10 21:57:15 X globalcollect_gateway_trxn: 29160570:8274273886 "2015-11-10T21:57:15+00:00" "29160570" "Gateway response: Original Response Status (pre-SET_PAYMENT): 600" "false" Nov 10 21:57:15 X globalcollect_gateway: 29160570:8274273886 Pushing transaction to queue [complete] Nov 10 21:57:15 X globalcollect_gateway: 29160570:8274273886 Clock at Confirm_CreditCard: 1.2096 (1447192635.9241) Nov 10 21:57:15 X globalcollect_gateway: 29160570:8274273886 Displaying thank you page for final status complete Nov 10 21:57:15 X globalcollect_gateway: 29160570:8274273886 Redirecting to [/index.php?title=Special:GlobalCollectGateway&appeal=JimmyQuote&ffname=error-cc&gateway=globalcollect&country=US¤cy_code=USD&amount=25&language=en&utm_source=sp49981691.default%7Edefault%7Edefault%7Edefault%7Econtrol.cc&utm_medium=email&utm_campaign=C15_Email1&payment_method=cc] Nov 10 21:57:20 X globalcollect_gateway: 29160597:9157669488 Attempting to set a valid form for the combination: {"amount":"25.00","ffname":"cc-vmad","country":"US","currency_code":"USD","payment_method":"cc","payment_submethod":"","recurring":"","gateway":"globalcollect","utm_source":"sp49981691.default~default~default~default~control.cc","referrer":"https:\/\/payments.wikimedia.org\/index.php\/Special:GlobalCollectGatewayResult?order_id=8274273886&REF=000000657082742738860000100001&RETURNMAC=y1DR%2F1BMpdFI3SwMtGs%2Fzi24ZcUE0FSNzzlgOie6QNQ%3D&AVSRESULT=Z&CVVRESULT=M&EXTERNALREFERENCE=29160570.14300"}