Have seen this for some time in OS X. If you are watching a stream from YouTube for example, and pause the video, and then resume it, it will usually stop after a few seconds and go back to the Playlist. Trying to relaunch the video from there may work or may produce an error that the resource doesn't exist.
I noticed this happens usually if you leave it paused for longer than a minute or so. I haven't measured, but I'd wager it's around 95% fail rate if you pause and resume the stream after a minute or so. My internet speed monitor shows (in the fail case) that no more data is being downloaded.
Pausing and resuming quickly (within seconds), the stream will continue without problem.
Would be good to know if this is a known issue that can be fixed or is being looked into. I've seen it happen for the past few years across versions.