I've stumbled across a few YouTube video lists in the past which were supposed to have soundtracks to a game or a movie but somehow had random videos in-between expected videos (only a few were like that). I wonder if it was a mistake or intentional, or if those videos (being soundtracks) got taken down and after several years were reassigned to new videos. Does YouTube ever reuse video IDs/urls?
No, I don't think YouTube ever reassigns old video IDs.
YouTube isn't running out of video URLs any time soon. In his video Will YouTube Ever Run Out Of Video IDs?, Tom Scott calculated that YouTube could generate over 73 quintillion video URLs. For reference, that's 73,786,976,294,838,206,4641 unique possibilies.
So while it's theoretically possible that YouTube is reusing video URLs, it is highly unlikely. Mainly because they don't have any reason to reuse old URLs.
1The number of unique URLs may actually be 18,446,744,073,709,511,616. Either way, it's still a stupidly large number.