[raluk]

In C / C++ there are two kinds of undefined behaviour. One is where there is written in standard what UB is. Another one is everthing else that is not in standard.

[thaumasiotes]

Technically, that's only one kind, because it's written in the standard that anything not mentioned in the standard is undefined behavior.

[cepepe]

One kind, but two different classes of undefined behaviour.

[wiseowise]

https://en.wikipedia.org/wiki/There_are_unknown_unknowns