I got an interesting case where somehow my compiler wrapper makes one particular preprocessor command invoked by embedjs.py fail. The error didn't propagate and the build continued happily, up until it failed during make package because the resulting xpcshell quits unexpectedly (it probably crashes). While a wrapper that fucks up is uncommon, the same kind of problem could appear if the preprocessor fails for some other reason, like a missing #endif in the input file, or something alike. And it's a hard to find in the logs.