One of my best performances happened because I got to the gig ten minutes before I had to go on, so could only get one drink in before going up onstage.
The things was, without the alcohol, I was so nervous that my legs were literally shaking, and I was convinced I was out of tune the entire time. I went off stage at the end about to burst into tears, when all of my friends came up to me and told me I nailed it. I didn't miss one note! I was so surprised. I didn't have the false confidence of alcohol, and had no idea that I'd done really well. It was... Strange.
I also do better at band practice when I am sober. But when there is beer, there is a drunken wilted. It's a shame :/