You know you're too tired to play any more poker when this happens.
I was in a low-limit game at Harrah's last night. It was about 1:15am, I'd been there for a few hours, tripled my stack, and thought I'd play another hour or so before heading home. I was in late position, four people had limped in and I joined them, as did both blinds.
The flop came out and we all checked. The turn card was the king of clubs. This shocked me, because I was 100% sure I had the king of clubs in my hand. I double-checked my cards to see how wrong I was -- I didn't have a king OR a club!
I folded and was in my car minutes later.