Hey,
Started using mobile payments by NFC today. It fails sometimes (or I have to get used to the exact position of my NFC chip), but the vibrating feedback you get when the transaction either fails or goes through is the same. This is particularly annoying, because (at least in the Netherlands) the NFC terminals are positioned sideways, so most of the times you can't check on your screen what's going on (you just keep om smiling to the guy at the counter thinking everything is OK).
My suggestion:
1. Correct transaction: One long vibration (which is now the case)
2. Failed transaction: 3 short vibrations