Different frame rates in one playlist

[CameOP]

Hi everyone

I've been browsing the forum and couldn't find anything about following issue:
I'm wondering how "safe" it is to put clips of different frame rates into one playlist.
I did quite some testing recently and it seems like the movies play each on their own frame rate, but I remember to once have had problems, such as the 24th frame being doubled when played on 25, which looks like a little stop every second. Meanwhile I guess it was the clip itself that was done wrong, but I can't verify as the clip has long gone.

Does anyone know anything about that?
Kind regards from Switzerland