This sounds wrong. You're probably thinking of undefined behavior that happened to behave the same across all available C++ compilers, so people began to rely on it. But since it was technically undefined behavior, optimizers were free to take advantage of it, and when they started to it smashed any code that relied on that undefined behavior behaving in a consistent way.
While technically correct, (which is the best kind of correct), the sad reality is that de-facto standards matter. Languages that understand this tend to be safer than languages that do not.
