Not the GP, but I have one non-partioning AIX trivia I discovered by accident. Turns out that malloc(0) returns NULL on AIX, but non-null on Linux, SunOS, and Irix. (I was surprised multiple ways on that one!)
killall is very different between Linux and Solaris, for example. I would rather have not found this out on a client machine for the first time.
Including the existence of import libraries, linker export files, and lazy dynamic loading of symbols.
^^^ THAT is how an Interview Question should work around this factoid.