Consumerist.com shares 10 Things You Might Not Know About Your Credit Card. Follow that link to read the full list, but the two that stand out to me are:

1) Unsigned Cards Are Not Valid And Merchants Can And Will Refuse Them
You might think that everyone knows that you have to sign your credit card in order for it to be valid — after all — there's a panel on the back that says "Not Valid Unless Signed," but you'd be shocked at the number of angry emails we get from people who have tried to use an unsigned credit card with "SEE ID" or "CHECK ID" written on it and were turned away when they refused to sign their card.

4) Merchants Cannot Require A Minimum Transaction Amount
It's a violation of the credit card company's merchant agreement to refuse a transaction because it is below the "minimum."

Side note: if you aren’t aware of what an absolute gold mine Consumerist.com is – give it a look-see. It really comes in handy should you be trying to escalate your horrendous customer service experience up the corporate chain, or read up about other folks’ experiences so you’ll know how to avoid the latest internet or consumer scam.