It's really amusing (in an oh-my-God-I'm-fucked way) to try and debug type errors in Happy-generated parsers. There have been times when the type of the function (not even the whole error message) has been a page long.
Intel's C++ compiler produces much more readable error message, I find. Though, almost perversely, I've gotten used enough to GCC's cryptic template error output that I'm more comfortable with it.
C++ STL code can be similarly (un)fun.