It was the freaking neutral. Opened the panel, traced the hot down from the breaker- started tracing the neutral up and immediately heard it sizzle. It wasn't just loose, I'd never even given the screw the first turn. ARRRGHHH! I'd even gone through that panel a month after it was installed and torqued every single screw and STILL missed it.
Ah well, I suppose the good news is that my house hasn't burned down and it was easy enough to snip off the damage a pick a new slot on the bus bar.
