Hitfilm 3 Pro Mask / Sharpen Bug
Here's a sneaky one. Even trying to film it took a few goes, as it's not consistent, but is eventually repeatable. It is also present on the output video once it does occur, so it's not just an editing render bug.
Basically, adding a Sharpen effect to a video with a Mask produces a weird colour clipping line at the edge. Sometimes. Looks different in 16 or 8 bit modes.
Depending on the position of the mask, and also depending on whether it was moved before, or after, the Sharpen Effect was applied, variations are:
1) No line at all.
2) Lines on one side, not on the other, or the top/bottom.
3) Line on both sides, not top/bottom
4) Line on the bottom, not on the other 3 sides.
As the video shows, inverting the Mask makes it disappear, as does turning off Sharpen.
Also, moving the Mask itself after the lines have appeared can usually make the lines disappear, but not always, which is why the video doesn't show all the variations, just the one I managed to capture cleanly and briefly.