I, too, have experienced the “goblin in the machine” a few times—we get a good mix during soundcheck, and have it through the first set, and then once we get on stage for she second set, BOOM! Instant horrible mix!
I used to blame the sound guy. But, I have been the sound guy in that situation before. It even happened to me/the band I was mixing this weekend. They got done with their first set, and took their break. I took my break, and had a chance to eat the meal that was brought to me, so I was at the soundboard the entire time, and not. a. single. thing. was. changed. When they started their second set, they complained about the mix, and asked why I changed it. I told them I didn’t, but I would gladly make any changes they needed, so that’s what the first couple of songs were about. Basically, what they asked me to boost during soundcheck was too loud for them the second go ‘round.
I assume there’s a little bit of context to account for. When you soundcheck, you start to figure out, “I need some of this, and some of that—ooh, could I have more of THAT, please?” and you roll with it. When you leave and come back to the stage, you lose the context of what you’re currently hearing vs. what you heard before you asked for “more of X and Y” in the monitor.
I dunno. Just my thoughts, since I’ve been on both sides of the board when it happened. I’m still trying to figure it out. As long as the band and sound crew are working together, everyone wants the same results—there’s no need to ever get angry and confrontational about it. I never think, “Oh, the sound guy is out to get us! That’s why he’s purposefully making our mix sound bad.” 🙄