Funniest time it happened to me was probably when I drove off with my wet suit gear bag on the roof on the passenger side of the car. Some fellas waved me down, and one of them grabbed the bag off the roof, and when he landed it in the passenger window he said "stay off the ole weed". I also once forgot my mother's notepad+ on top of my car roof. I didn't find it when I realised and later went back.
Long ago one of my buddies left a case of 4 and 8 tracks on my car roof. We never went back for it.
There was a guy in my hometown that came up with a brilliant ruse to fool people. He glued a coffee cup on the roof of his car above the drivers seat!!! Whenever I saw him, people were waving frantically at him and he just smiled and waved back!!! I heard later that a cop stopped him and made him take it off the roof .
Ok, this wasn't on my car roof, but I'm going to post it anyway. When I was working around my home town in the 90s and early 2000s, I had a nice 12 foot long trailer. The front was sturdy metal and I kept a bowling ball in the trailer when it was empty or almost so. When I'd stop at a stop sign , the ball would roll forward and make a LOUD BANG and I must admit that it was fun watching people / pedestrians JUMP and look around when it hit!!
When I was a kid, my mom placed a tray of my favorite cookies on the roof of the car... I cried when I saw them slide off and crash all over the road. She hadn't let me have one before we were going and said I could have one when we get to our destination. I don't remember the destination but I remember that I lost out on my favorite cookies.