Which is what I'm involved in right now with C++.

Cross-platform issues:

1) endian problems
2) word boundaries (ask me about -malign-double sometime, *sigh*)
3) C++ differences between compilers (STL, mangling, pickiness, etc.)
4) Signals, threading, select, non-blocking sockets, etc.
5) all the other little things I've fixed that I've forgotten

Note that only #3 is specific to C++. But #3 is an ongoing, huge issue in my porting efforts.