Actually, I have changed, and I've changed corporate C/C++ MANY times to satisfy compiler upgrades. It is always just shitty code. It's never something insidious. Bugs happen. It probably wasn't your intention, but you've made UB sound pretty awesome.
If you are mainly dealing with shitty code, which frequently contains an UB, then most UBs you've encountered should have been from shitty code. That doesn't mean all UBs indicate shitty code.
