I was charged £81,000!! for a hotel stay in Mumbai on my AMEX - at the Oberoi no less. The hotel did chase this pretty quickly though, as you can imagine. Points and zeros in the wrong place...

However, I was credited with the £81K both by the Oberoi and by AMEX. Unfortunately, one of these was later re-credited back. Not sure, firstly, how it could ever have been authorised and, secondly, how I didn't have a heart attack on realising it.