The classic don't admit anything trick, so you can continue to rip other people off who might not complain.
I had that with a dodgy parking meter. Took coins, no credit. Ran out of coins in the end, and still couldn't park. Naturally, I complained. Was told that parking meters couldn't take money without giving me time, and that someone had been to inspect it and it was fine (erm, don't think so).
Yet, despite being so sure that it didn't fail and they can't fail, they gave me my money back as a goodwill gesture.
I felt like writing back and saying 'if you're so sure, why the **** did you just pay me' but knew they couldn't admit in writing that the machines weren't foolproof, as it might just wind up being used against them later on.