Complex software usually has some undefined behavior lurking that at higher or even just different optimization levels can trigger the compiler to do unexpected things to the code. It happens all the time in my line of work. If there's an extensive test suite you can run to verify that it still works mostly as expected then it's easier.
This is one where I suspect we don't disagree. But "all the time" can have a very different feel between people.
It also used to happen that just changing processors was likely to find some problems in the code. I have no doubt that still happens, but I'd also expect it has reduced.
Some of this has to be a convergence on far fewer compilers than we used to encounter. I know there are still many c compilers. Seems there are only two common ones, though. Embedded, of course, is a whole other bag of worms.
