In the course of transferring film (24, or for our purposes, 23.976fps) to NTSC (29.97fps or 59.94 interlaced fields per second), we get what's referred to as 3:2 Pulldown. What happens is that we get three fields from one frame of the film, two fields from the next, and repeat this forever. So, for frames ABCD of film, we get AA AB BC CC DD of video.
If we then simplify that to "I" for interlaced and "P" for progressive frames, we get PIIPP. A nice repeating pattern you can look for to see where the pulldown cadence falls.
Then, riddle me this, Batman: if we add one more abbreviation, "D" for a frame that is an exact duplicate of the last one, how on earth do we end up with this?
IPPDIPPIPPIPPPDPPIPPIIPI...
I ran into this today, and I honestly don't know if it ever repeats, because I gave up. A combination of speed changes and pulldown, and maybe a glitch of a dropped frame or two?
It looks like it was a 3:2 pulldown clip that got edited into a 59.94i show, and then effected somehow, but exactly how seems a mystery to me.
(And yeah, I know I'm saying "effected" instead of "affected", and that seems wrong by the rules of normal grammar, but if I'm saying that an effect was applied to the clip, isn't that the proper "term of art"?)