I guess they're different for different interviewers.
The one I would use is something about undefined behavior. I mean, the fact that it exists and consequences. For example: "this program works, but if I add this line of code at the very end, the program crashes. Can be I sure that it crashed because of that line or some incorrect inputs to that line?"
In (almost) all other languages the answer is "Yes". Not in C++ or C. You may have a faulty program in the first place that just happen to not crash.
All other quirks and syntax can be picked up when needed. You can play around and investigate. But undefined behavior requires one to think and debug in a very different mindset. One cannot assume that a program that does not crash and gives a correct answer will do that if extended.