It gets more confusing when those two compilers are the same one. Compiler bug causes compiler bug elsewhere in same compiler, which causes bug in compiled imag

It gets more confusing when those two compilers are the same one. Compiler bug causes compiler bug elsewhere in same compiler, which causes bug in compiled image of some library function used by the compiler, which causes another bug in the compiler when the compiler is re-compiled again this time with the compilation relying on the wrong compiled version of the function rather than the correct interpreted one. This second compiler bug affecting something is what you notice first, and is your investigative starting point. For most people in computing, a side project is the only way to experience this kind of thing.