I have two top funny ones.
The first involves my sanctuary settlement. I discovered that whether through a glitch or intentional I can build prefab structures on top of houses. By doing this I basically created a massive fort on top of the main house in sanctuary (where the power suit station is). One day I came back and discovered that bhramin had takenover my town. There were around 5 pack bhramin walking around and two on top of the house. There was one more attempting (quite pathetically) to climb the stairs to my fort. So I had 8 pack Bhramin with their massive packs all over, it was very funny.
The second glitch involved a legendary mirelurk and a quest I won't spoil. I was up against many creatures and attempted to hide inside a small building with a window I could shoot out of. Somehow the behavior for the legend mirelurk changed and he began behaving like an NPC. First I noticed that the door to the room opened. (apparently mirelurks can open doors or this was a glitch).
The odd thing was the mirelurk didn't come inside. I glanced out the door and noticed a car probably 10 meters away. The mirelurk was hiding behind the car and peeking its head out of the corner like people do when they are shooting at you. It wasn't attacking, it would just hide then peek its head back up for a second like a creeper.
I decided to play and blew up the car. The mirelurk hauled ass away and I couldn't find it again, it was bizarre, totally NPC behavior.
Those are mine, what do you guys have as your funniest glitch?