I have an example of a bug I found that looked like an optimization bug but was really a user bug. Someone had the bright idea of sprintf-ing to a string and using the string itself as one of the format arguments to the sprintf call. When compiled with gcc the code would still run fine, but when compiling with -O1 the string would end up garbled. The problem (and advantage, performance-wise) of C is that most of the time you do something wrong, the behavior is undefined, whereas most higher-level languages will spend the CPU cycles to protect you from yourself.

Another example I can't remember the details of, but it was related to the fact that gcc adds code to zero-initialize your stack on first access unless optimizations are turned on. Code that checked for null pointers worked fine until optimizations were turned on, at which point it was discovered that a variable was being used uninitialized.