Once, I accidentally bought a Spider Web for 10 time the amount. I needed to buy a lot, was chatting with someone at the same time and there was a bit of lag around the AH.
On the other hand, someone did pay me 200k for something that was 20k once too. I contacted the person to give him his gil back, and he said I could keep it. Yay!
So... yea.. that was it...