I found a bug in my copy of autocompile.py, where there was an illegal call just at the end of the compilation process. After fixing that, further attempts to compile the same package gave:
Build metadata file '/Users/dave/.config/0install.net/0compile/builds/http:##ryppl.github.com#feeds#boost#quickbook.xml/quickbook-1.49-post-201205092147-x86_64.xml' already exists!
But I've seen this symptom a lot. Something needs to clean up these directories after compilation is done, I think.