I think it would be fun to collect studio gaffes. Recording is complex, and we all start from zero and figure it out as we go. We're bound to fail sometimes.
Here's one that I just discovered recently when reading some old session notes from 10+ years ago. The note said "Audix D1 under the snare -- facing exactly away from the kick drum to help isolate the snare signal." That's a gaffe because the D1 has a hypercardioid polar pattern. What I apparently didn't understand at the time is that the HC pattern is essentially like a baby figure-8. Hypercardioid mics pick up more sound at 180 degrees than Cardioid mics do. That means that facing the mic "exactly away" from the kick drum was the *worst* way to minimize the kick drum sound in the bottom snare mic. (The nulls on most HC pattern mics are usually about 30 degrees off that rear center point.)
Too esoteric? I guess I could share the time I tried to re-amp a snare drum... put the studio monitor on top of the snare, pointing down, with a mic underneath the drum pointing up. I thought I'd play back my pre-recorded snare track and try to record some snare response because I hadn't tracked a bottom snare mic originally. No problem at all... except the mic's output wasn't muted in the DAW. The mic was playing back through the speaker on top of the drum. The feedback was horrific, for about half a second. And then the mic died. Not kidding. That was a $250 mistake. But the sort of mistake you only make once.
Anyone else willing to admit a mistake from a past session? No judgments here -- I've made plenty.