HotSpot certainly does better than the vast majority of JITs out there, but it is definitely not perfect. If you have a JIT cache that is too small (really easy to do if you're not aware of the JIT settings or what they do), or your code does most of its heavy lifting in functions that are not called very often (because call frequency is the only heuristic it uses to prioritize optimization), you can easily trap yourself into horribly optimized executables that the JIT doesn't know its way out of.