I recently came across a rust book on how pointers aren't just ints, because of UB.

fn main() {
    a = &1
    b = &2
    a++
    if a == b {
        *a = 3
        print(b)
    }
}

This may either: not print anything, print 3 or print 2.

Depending on the compiler, since b isn't changed at all, it might optimize the print for print(2) instead of print(b). Even though everyone can agree that it should either not print anything or 3, but never 2.