So whats up if I didn't sign a credit card receipt?

[Rock]

So I started a tab on my credit card at a bar the other day. And paid out cash. I didn't sign anything and specifically told the bartender that I was going to pay cash. Anyways, I didn't get a receipt for my cash transaction and today I noticed that I got charged at the bar on my check/credit card.
So I'm wondering if the receipt doesn't have my signature on it, are they allowed to charge me regardless, or am I fucked because I don't have a receipt for paying cash?