Here's a fix to a random annoying message I was getting on my iPhone, in the Photos application...

When entering the application, or scrolling between videos and photos, I would get a pop-up message stating "This movie format is not supported"... even when viewing photos. After some not-so-furious Googling, it was solved by scrolling through until I found a video that would not play - it had the video icon and play button, but was all black. Deleting this corrupt video file fixed the issue...