I confess...

Richer Sounds once accidentally charged me £10 for a £350 pair of speakers and I didn't tell them.
Forbidden Planet, twice, charged me £16 for £160 worth of goods. Again - I didn't tell them.

I still don't know if I would tell them. Their fault for being idiots.

I should have got a 15% discount in Halfords, the assistant misheard the manager and gave me 50% off. This was on a £750 bike 15 years ago. I am yet to feel guilty.