We’ll use this thread to discuss / explain new and weird phenomena in vessel movies and/or peculiar movie artifacts.
In the current, “Long Term” dataset we have a large number of movies presenting with such “blackout” areas, that some catchers asked about on the chatbox:
https://stallcatchers.azureedge.net/sc-movies/r/az_9f82b14f4675421a8a35770debfc06bc/az_9f82b14f4675421a8a35770debfc06bc.mp4
These are due to a new approach to reduce the incidence of “bad” movies by blacking out the dura layers, which contain vessels much larger than capillaries that would not be affected by leukocyte stalls. In fact, right after the blackout disappears you can see there was a large vessel behind it. This process (“masking”) is done manually in the lab, and basically looks like someone drew on the movie with a black permanent marker, which is basically it 
And, of course, there can be some “masking errors” as well from time to time.
Most of these “blackouts” will not affect the outlined vessel itself though, so you don’t have to flag them. Just evaluate the outline vessel as usual, the best you can.
Here is another example:
https://stallcatchers.azureedge.net/sc-movies/r/az_1cb8be2f54ba40ceb6af659696472310/az_1cb8be2f54ba40ceb6af659696472310.mp4
This one is probably a borderline case where you can barely see the outlined vessel (it’s only visible for a couple of frames when the blackout disappears. You may of course flag it if it’s particularly difficult, but even so, do the best you can to guess whether it is flowing or stalled, and just move on to the next!