This has happened more than once. I need to power phone off to fix it.
Running Lollipop
Watch a YouTube Video, finish it, close YouTube, even end task and verify it isn't running. When phone locks the preview image is visible for the video on the lock screen. :-\
Running Lollipop
Watch a YouTube Video, finish it, close YouTube, even end task and verify it isn't running. When phone locks the preview image is visible for the video on the lock screen. :-\