The big boss is unpredictable and she definitely blows hot and cold, lol. Well...for what it's worth, I'd be leaving the company because I'm relocating, not because of work related problems or another offer.
I started this job knowing next to nothing (it was a "no experience necessary" corporate job -- rare, I know), so obviously someone else could too. The thing they might be pissed about is actually having to seek out a replacement, but ultimately that's not my worry, especially since this isn't my field. I certainly didn't expect to work here forever... I just hope my immediate supervisor will be a reference if necessary.