Ponies corrupt all, only a few like me can see their true evil and thus don't fall into the trap.
But I'm a Windows user and a Memory Corruption seems a bit much for a simple BP addition.
Yeah, that was confusing me too. After I went to sleep (that was past midnight here), I figured out that it's nothing to do with the part at all: I'm adding the body part to a specific caste by placing that BODY line in that caste, and if I add a BODY line that's a duplicate of the base definition it also throws the same exception. Probably should have mentioned that was how I was adding it, really...
EDIT: Yeah, got it. Was defining body template and material properties before all the caste definitions, then attempting to modify the base body pattern. Unsurprisingly, it didn't like that. So now I do body materials at the bottom and body definitions per caste, and it works fine. Basically, I'm an idiot.