let's see...there are several kinds of *nervous* maybe (that I have experienced). The first one doesn't apply to what I was really saying but is worth mentioning. Back in my reel to reel days, I worked on a 16 track hybrid MCI/Ampex machine that basically had lousy brakes on it and if you weren't careful, that tape would like to streach on you during rewind/stop. Fortunately that never occured to me at the worst possible time..but always kept me aware and thinking about that...and nervous on playbacks late at night, after all the takes where recorded. Some great stuff recorded on that machine, but I'll always remember it for its shortcomings...and our lack of $ at the time to get it fixed...or find someone who even could help us fix it.
However..to paraphrase...maybe just the antsy-ness of trying to capture a [first] take (no official levels gotten maybe) of some mind blowing players, and/or just making sure everything was in sync from the machines being in record, to no other nasties etc.like an amp buzz, hum, etc. I must admit too, that being in the presence of some killer musicians sure makes me be a tad squirmy (but I'd never let it show ;-O)especially if I am just engineering and not playing. ...kinda like they are wailing, so I better too, with my side of the responsibilities. The problems don't seem to stem with the audio quality, but rather with issues like; going back to bar 137 in the middle of the improvisational free-for-all, in some Chick Corea type mind f**k, to punch in the three bad notes I never even heard...even on playback ("so where do I punch in?") And the bass player even thinks it was obvious too!! har!!