This applies equally I think to BP - I found out something new. You can't attempt more than 8 cards.
I had a situation where I left the mall with 22 cards, knowing only 20 were active, and didn't worry much about which 2 were duds - figured they'd kick out. While doing a 4-swipe transaction I think I hit a dud, then reached into the "used" pile in front of me, by the time I un-confused myself I'd tried 8 cards and now nothing would work.
It wasn't a big deal - we just cancelled and started again with known good cards, and the money pulled off cards was restored by the next day.