Why won't some highlights finish saving?

I like saving my highlights that I think are at least notable, if not pretty good or great. However, on two instances so far, the progress bar for saving the highlight has completely stopped, and there doesn't seem to be anything I can do to recover.

I can hit Escape and click the cancel button, but that doesn't do anything; I'm still stuck on the screen with the progress bar. No other key press does anything either. I basically have to force quit out (Alt+F4 works) and reload the game in order for me to do anything.

I've also tried lowering the settings of the recording, but that didn't do anything either. It still gets stuck.

This issue shows consistent behaviour with particular highlights. It always seems to happen, and it always stops recording in the same spot. Just playing the highlight doesn't do anything unexpected, though; it plays start to finish just fine.

Why is Overwatch doing this, and how do I fix it?

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Tonight is the third time this has happened, and I went even further reducing the quality of the video:

• 1080p (this is my normal setting)
• x1 super sample (usually x2)
• 30 fps (usually 60 fps)
• low movie quality (usually ultra)
• auto movie format (still happens if I select WebM explicitly)
• hardware encoding off (normally it's on)
• music on (not that it should matter in this case)

The only thing that seems to be common between the various highlights is that they tend to have a lot of particles/effects.