Most people seem to think it's related to panning or motion, and it's not. MPEG-2 handles that reasonably well. It has to do with the percentage of the screen that's changing, frame-to-frame.
Where you're going to see the footage break down is when everything in the frame changes, in an unpredictable way. Splashing water, or a rippling pool of water... fire will do it too. Zooming while panning can make it happen. Shooting a long telephoto shot, where there's heat waves rippling through the frame will do it. Backlit sparkly water like a lawn sprinkler can do it. Shooting shots where there's a lot of fine detail, like lots of blades of grass, can use up the bitstream, so an unexpected event in the middle of that can cause the issue -- a relatively static shot, for example, and then the sprinklers come on...
Where MPEG-2 shines is when there's little changing in the frame. So if you're shooting a beach shot of 80% sky and beach, locked-down, and the only rippling water is a little band in the middle, well, MPEG-2 can probably handle that. Change the shot to where there's nothing static, but you're shooting down at the waves and the screen's full of splashing waves, and it might choke horribly.
The more that changes from frame to frame, the harder it is for MPEG-2 to keep up with it. You'll notice on HDV demo shots from the manufacturers that they're usually locked-down shots; that's because in a locked-down frame there's a lot that doesn't change between frames. MPEG-2 encodes the changes between frames. Try filming a horse race like the Kentucky Derby, where there's horses charging hard around the track and the camera's wildly panning to keep up with them and there's a whole lot of action going on where their hooves are and there's a cloud of dust flying up -- MPEG-2 doesn't like that one bit. Horse racing is hard to watch on broadcast HD, it just totally falls apart and the whole legs/hooves area becomes a big blob of macroblocked ugliness surrounded by an apocalyptic swarm of mosquito noise.
Those are just some suggestions. Other areas where broadcast MPEG-2 falls apart are in things like the swimming and diving competitions at the Olympics, for example. Or a basketball game -- basketball is extremely artifact-y, although less so on ABC 720p than it was on CBS 1080i. Panning and zooming and a lot of action and a lot of detail = a severe challenge for MPEG-2 GOP-based compression.
I hope I answered your question...
