OK - you experienced guys have seen a lot, and probably aren’t surprised by anything. I’ve been fixing things for a long time, and have seen alot due to neglect, damage, d-i-y’ers that weren’t very handy, etc., but have stupidly had an assumption that it was right when it started life.
Here’s a couple of pictures from a mock inspection I’m working on that blows that theory out of the water. House was built in 1996.
First picture is return grille with a door jamb installed on top of it (no one will notice). Second is a supply vent for a toe-kick register - supply house must have run out of boots).