Are you guys absolutely sure this couldn't have been processing error? I just processed some HP5 and the same thing happened - only now it was with a roll shot on my Nikon F90. The frames look very dark, just like the ones on Page 1 with the Hexar, so I'm guessing they'll be nice and bright.
I'll have to scan tomorrow after it's done drying to investigate further, maybe that scene in particular was bright. But based on the fact that it was semi-indoors and from what I can surmise from the actual negs, it is no different from the correctly exposed frames, I'm guessing it is an exposure issue.
I also shoot manual exposure on all cameras but the Hexar, so in this case it may have been me forgetting about the meter. Again, will have to look at the positives on the screen in order to know for sure. But in the meantime, I just want to know if there's any way it could've been processing error?