You're absolutely right. It's not even just optimization. If you exercise your C++ compiler hard enough, you will find bugs. You have to know what the compiler is doing.

I have the misfortune of working somewhere where I generally have to use older versions of compilers for production. I've found two compiler bugs just in the last two years since I started my current project. I'm usually the last person to blame the compiler, but in this case I can be pretty confident because the bugs I found were fixed in later versions of the compiler.