[clivehawkins]

The first one I can remember is when I went to the bank to cash a cheque. I wanted to say "Vorrei incassare quest'assegno" which means "I'd like to cash this cheque".
What I actually said, and there really is only a very small difference, is "Vorrei incazzare quest'assegno" - "I'd like to really piss this cheque off!"
Top marks to the guy behind the counter who managed not to laugh in my face and politely explained the difference in pronunciation.