I did that with a cup of coffee once when I fueled up. I got down the road and realized I didn't have it with me. I drove back and went inside asking the clerk if I left it there. She told me she saw me set it atop the roof of my car and when I looked out by the pumps the fresh spill was evident.

 

i left a pipe wrench on my rear bumper after i took off my trailer hitch. rode 300 miles to work and found it still there.

i saw a pair of channel locks on a guys bumper that looked like they were going to fall off. i told the guy while we rode down the interstate. he pulled over and got them off.

 

I roll my own smokes, and have a box that I keep the 'stuff' in..... Left it on the roof once, realized as I was driving down the road, that I had one rolled in there, that I wanted...... and remembered the box.... as I heard it slide off, and hit the road...... It was somewhat worse for the wear.....

When working on vehicles for a living..... I would find all sorts of stuff on customer cars.... My personal favorite was a truck that came in with a brake pull after changing calipers...... Rolled the truck into the shop, put it up on the hoist to inspect, and found a pair of needle-nosed vice-grips clamped on the passenger side brake line...... Took 'em off, took the truck for a test drive, and strangely enough, no more brake pull..... I wish I had a picture of the customers face when I handed them to him, and told him the truck was fixed.

 

My rear bumper things always fall off, but my front bumper(gotta big steel one) things rarely fall off and in fact i often carry shovels in it lol.)..

Wrenches
screwdrivers
pliers
soda
..radio
pager..(both gone =S)
fire helmet on hood...(was on a fire 72hrs and was soo exhausted i didnt see it until i stopped in my driveway , and it made it home!!!)

 

Let's see: One Timex digital watch, broken. One unused prophylactic, one soiled. One black suit jacket, one pair black suit pants. One hat, black. One pair of sunglasses. $23.07.

 

I have not left anything on the bumper for a few years, it seems that I like to now put coffee, wax bottles, rags, cell phone,work phone on the top of the tire forget about it and then drive over it.

 

Cell phone, sunglasses, soda bottles, hoodie's, rags, cleaning bottles, wrenches.... way to convenient lol.

Best one yet to go with HeyYou's....
Guy I work next to here in the shop does an oil change one day on a GMC truck. And later in the day could not find his oil filter wrench the truck came back 6 months later... and there it was sitting on the plate that goes under the oil pan lol. That was funny.

 

I've found a real nice pair of Snap-on Needle Nose pliers wedged behind the airbox of my mom's Camry.

I'm guilty of leaving a claw hammer on the driver's side inner fender of the Dodge. That night driving a strange popping noise apperared after driving that night. Strangely the hammer had wedged into the coil spring. Was fun getting that out. Should have used a Ball Pein.

 

Jug o kool-aid, oil, tools, smoking tool(got that before it broke lol) lug nuts, cd's, cell phone, and I left a hot pocket in-between the intake manifold and the valve cover for a week.

 

It's good to see that I'm not the only one that does it.
Now a hot pocket is what stands out the most to me, were you trying to keep it warm or something? I tend to put food beside the lights on the brush guard if I do end up eating while working on the truck.