I think his problem is. He tries to delete/backspace some characters but instead of deleting them, the n900 add characters or commas, on a web form (e-banking). This sounds like Num lock to me on a laptop keyboard (the one that has no physical numLock, but actually you have to press Fn+Numlock to get the numPad on the side).

I don't yet have a n900 so I can't say if this is correct or not, but I'm more than 50% sure this is the case.