Hmm, so maybe it only happens after you build the 64-bit version first. But apparently it's more complex than that, since that doesn't repro either. When I get some more time I'll run though and double check all the rebuild conditions, but it has worked fine for me every time I've done it across 8-10 machines and 3-4 VS versions.

I think you've ended up in a very unlikely situation that is probably going to be fixed by "hg purge --all && PCBuild\build -e -d && PCBuild\build -e -d -p x64".