I once ordered black pudding and received a two pack of chocolate sponge puddings. I think it was Tesco. That was genuinely funny though, and in all my experience has been good.

Also, call me a frugal jerk, but I spent about 2 years adding dots to my Gmail address to get £15 off as a new customer offer every time. It got a bit awkward when the same delivery driver had to give me the 'new customer' spiel and said he was sure he'd been here before.