I found a way to replicate this pretty consistently.
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Start video playing, for testing I used https://www.youtube.com/watch?v=RjtCS0EEoCY
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Scroll down & expand some comment threads by clicking some “view all X replies” links
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Scroll back up and full-screen the video
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Video stops and re-blocks, and can’t immediately be unblocked because the button isn’t on the screen.
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Exiting full-screen, you’ll be able to unblock the video button the button will be in random strange places.