Pointer errors. Null pointer returns instead of using the correct types. Flow/state problems. Multithreading problems. I/O errors. Network errors. Parsing bugs.

Pointer errors. Null pointer returns instead of using the correct types. Flow/state problems. Multithreading problems. I/O errors. Network errors. Parsing bugs... etc Basically the whole world of bugs introduced by someone being a too smart C/C++ coder. You can battletest parsers quite nicely with fuzzers, because parsers often have multiple states that assume naive input data structures.