This one takes the cake

This is a class B vent on the furnace in the attic someone wrapped the vent with a piece of black insulation that is normally used on the interior of rectangular duct. This insulation has a glue sprayed on it and is not plenum rated. They were trying to keep the plastic on the flex duct from making contact with the flu.

And another primary A-coil drain headed to a vent stack???

Hard to tell in the photo,but are all those 90’s creating a trap?

The heat probably made it more pliable. ha