the only time this has ever happened to me is UW cardiology fellowship interview where they are sort of famous for doing this. I interviewed broadly for residency and didn't hear of anything like this happening. I wouldn't worry about it.
It is not uncommon to get "tell me about an interesting case you had" during interviews, I got it 3 or 4 times (out of 16 interviews). In that case, they want to hear something that makes it clear that you're actually interested in the specialty and have put some thought into your choice.
In these cases, you don't need to give a case presentation at all. Just talk about the patient you saw on X Service who had Y Disease and you thought it was interesting/educational because of Z (presentation, exam/lab/rads findings, treatment, whatever).
There are a few programs out there who will ask you to actually present a case semi-formally, but they're rare.