I have bumped into this myself, too. It's really annoying. The biggest footgun isn't even discussed explicitly and it might be how the error got introduced - it's when the struct goes from POD to non-POD or vice-versa, the rules change, so completely innocent change, like adding a string field, can suddenly create undefined behaviour in unrelated code that was correct previously.
wow, can you elaborate how adding a string field can break some assumptions?
Not the OP, but note that adding a std::string to a POD type makes it non-POD. If you were doing something like using malloc() to make the struct (not recommended in C++!), then suddenly your std::string is uninitialized, and touching that object will be instant UB. Uninitialized primitives are benign unless read, but uninitialized objects are extremely dangerous.
That's not what was happening in this example though. It would be UB even if it was a POD.