I quite often forget to double check the gain at every stage and end up with clipping drums that I have to re-do! Also drummers tend to knock mics - don't forget to move them back! Turning headphones on with the volume set insanely loud. Forgetting to tune everything to a digital tuner. Forgetting to tune the drum kit!
One I do quite often which might be specific to me, is come back to my studio where everything's still set up, and plug my PC in. I plug in the soundcards the wrong way round, and record snare onto the vocal channel, kick on to the bass, no vocals etc... just because I get two plugs the wrong way round and don't double check before hitting record.
Another mistake we make quite often is getting drunk during mixing.