Exploitable non-obvious source code back doors.
Can't I just obfuscate?
Backdoors that are pure obfuscation need not do a pull request... if you have some code that would make a neat backdoor, please send a pull request. I prefer things that are really hard to spot or attack the compiler.
The backdoors I have here cause sig11/access violations in compilers for instance when their not precisely formed.
That's the sort of thing I think is cool, obfuscation is too much a variation on a theme, though it can be used to vector a cool attack.