I think relaxing too much because everything's going great can also be a big culprit.
We played a small festival on Saturday night, (our first ever) everyone brought their A game, concentrated on what they were doing, and we had probably the best gig of our (granted somewhat short) career.
The following night, we played a local charity gig which we've done before, to a handful of punters and were all relaxed as it was a throw away gig. It was bloody awful! So much so, the drummer started packing her kit up in the break and had to be talked around, and I wasn't far behind.
We had a word with ourselves in the interval, and the second set was spotless.
It was like playing with two different bands.