As we mentioned earlier, when you purchase an HD TV show from iTunes you're actually getting two copies of the show -- one version in HD and another in standard definition for your iPhone. They're two separate files and, well, apparently they don't always go great together. Ars Technica is reporting that when iTunes gets around to downloading the SD version, some users are finding that it's overwriting the HD file.
They recommend turning on "Allow simultaneous downloads" and also offer a sneaky quit-and-restart iTunes trick. If that doesn't appeal to you, just sit tight, Apple's aware of the issue and will likely manage to fix it up (here's a hint, Apple, just append the resolution of the video to the end of the filename).
How about it: are you downloading HD content yet?