From what I've seen, many of these behaviors appear because the semantics of the language are too low-level for the things that people are doing with it. It's not that compiler writers are maliciously optimizing things into nonsense, it's that there are things that clearly need to be optimized.

One example that comes to mind is looping over each element in an array. C doesn't actually have this concept in the language. Instead, you use an idiom that's so common that you'll read it as looping over an array:

    for (int i=0; i<n; i++) { do_something_with(arr[i]); }

The compiler can also read this common idiom and find out that it's going to be doing a bunch of sequential memory accesses, which it can optimize. That's great. Except, what if the pointer arr is near the maximum value for its type and it'll overflow when you add i times the size of the array element to it? Then you're doing some very non-sequential accesses.

This doesn't make sense in context: you wouldn't have an array that's split across the end and the beginning of addressable memory. You'd rather ignore that case so you can optimize the really, really common case of iterating over an array.

But a better-designed language can give you a way to just say "for each element in this array".

Rust aims to be as efficient as C, but does not have the goal of letting you directly reuse C code. Its surface of UB is much smaller than C's, and easier to define, because the language is more expressive of what you actually intend to do.