I try not to sign anything that isn't 100% correct.

I was buying a bus (*ugh*) ticket and the agent ran my credit card for two tickets accidentally. She gave me the slip to sign and I refused. She said "you have to sign it before I can cancel the transaction." I said, "No, I don't. Just cancel it."

This went on for a couple minutes with both of us getting aggravated. Why the heck would I sign an incorrect receipt? Finally I put an end to the argument by taking the receipt from her and writing in big letters "VOID" across the signature line. She wasn't really paying attention and thought I signed it had -- boy was she pissed when she saw that I hadn't

She did give up on trying to get me to sign the receipt at that point, however, and just went ahead and voided the transaction, like she should have in the first place.