I don't buy the argument from authority, I really think you're trolling.
Apart from the bad FPU code in gcc 3 (or generally relatively poor code in gcc 3 compared to more recent or more expensive compilers) I really don't know examples of significant problems in the code generation of modern C compilers. Just don't give me examples that work only on the latest CPU's or that are fast only on one of CPU generations or on the CPU of only one manufacturer.
I don't have the time to provide examples I'm afraid, so take it or leave it ;)