I work with a lot of different mp4 files - I always convert them to ProRes 422 before working with them in FC.
Every once in a while, though, I have the hardest time editing them. For instance, if I drag the ends of the clip, I can see the video in the canvas, but if I try to just play or scrub through, it's just black. If I put it into the viewer, I can't see it at all when I play. I always use the same settings, so why does this happen sometimes? (And by "sometimes", I mean maybe 1% of the files. It doesn't happen very often.)
I wish I had an example right now, but I can't remember the last file that did this. I just happened to be thinking about it as I converted some h.264 files (which converted beautifully) and thought I'd post before I forgot again.