Attack vectors introduced by compilers
Not something I have a real life example of, just an idea.

Certain computationally intensive tasks can be done much faster using memoization. Now if the compiler manages to automatically use memoization for some piece of code that code might run much faster than what the language notation suggests. I'm not aware of any compiler actually doing this though.

In C++, for example, any optimization is allowed as long as observable behavior is preserved which includes accesses to volatile memory and calls to I/O library functions. The compiler is allowed to transform the program in any way otherwise.