Technically the framerate does not even need to be constant! It's not rare to have minor fluctuations for effect; though following this thought through one could argue that slow-motion segments are actually higher framerates.

The question then becomes are we talking about the raw footage or the processed clip? Then we also need to ask about different regions and systems, that can have different standards for output. Then the source material could be one framerates, and the output could be another!

... What was I doing again?