NOT FIXING IT IN THE MIX
Sometimes, while recording, we're so happy just to be getting sounds to tape ("Hey! This thing actually works!") that we forget that we're just on the first step.
Once we know that we've got signal, that we're actually modifying bits and creating data, we have to stop and listen to what we're getting. This can be the hardest thing to do, especially after you've spent hours (or weeks, or months) just setting everything up. It seems like the hard part should be done. After all, now it's up to the band.
This is a mistake. The real job doesn't start until everything is already working. It should be a given that the meters are all moving, that the lights are blinking, and that we're not distorting. But the heady rush of the record button first being hit can be followed by a bit of laziness, by figuring that our job is over until playback.
Remember to keep listening, from the first note to the last fade out. Something not sounding right? Adjust it. Getting distortion? Turn it down. Think you might need to move that mic? Go do it. Sometimes I warn bands that, at least at the beginning, I'll be coming in and out of the live room as they play. Some recording engineer once said that he's never worried about screwing up a take by moving a mic. On some of his recordings he claims you can hear adjustments being made as the band plays. I'm not sure if this is something to shoot for, but if something sounds shitty at the beginning then it should be fixed by the end.
Of course, that's kind of a British way to think.