Whats the funniest job site prank you've seen or heard about?

At lunch, when everyone took off their toolbelts to eat, my buddy would go and nail the new guys belt to the deck (he'd nail it so that it was hard to get the hammer out).

I remember when I was new and cut a stud too short, the lead carpenter told me to go get the board stretcher out of the truck. I felt really stupid when I came back saying I couldn't find it, and they laughed.

Answers:
not jobsite pranks.. I went to a voc-tech. The few pranks we would pull on freshman:

Bucket of steam
Left handed screw driver
Paper stretch
Lathe/chuck mover

Dangerous one: someone got injured o over there and weld that hole in the empty gas tank. Unsuspecting kid lit up the tourch put it close to the hole in the tank which followed fire and a small explosion.

those are just a few

My husbands biggest prank was putting ketchup packets underneath the toliet seat where the bumps are the hold it up, while he was in the military hoping to get one of his buddies, well major walked in and used the bathroom, satdown, and got ketchup all over his uniform. My husband confessed to the prank and had to pay for his dry cleaning. Thank godness, it could have been worse.

Other answers:
When the plumbers screw with us we put gunpowder in their flux.
When the new guy is just standing around, we toenail his shoes to the floor with the gun.
We screw lunchboxes to the floor.
When the new guys keep missing nails with the hammer we tell them, now that it's surrounded, go in for the kill.
We make new guys look for the concrete softner.
When the plumbers screw with us we put gunpowder in their flux.
When the new guy is just standing around, we toenail his shoes to the floor with the gun.
We screw lunchboxes to the floor.
When the new guys keep missing nails with the hammer we tell them, now that it's surrounded, go in for the kill.
We make new guys look for the concrete softner.
my aunt had a clothing store and my uncle painted water towers, bridges, etc.. one time my uncle was painting a water tower and his foreman would go down to the local pub and drink half the afternoon, then come back at quitting time.the water tower they were painting had alot of small pine trees around it, where you couldn't see the lower section of the tower, so my uncle got a mannequin from my aunt's store and dressed like the guys and had it about 40 feet up the tower. when the half drunk foreman returned from the pub that day, my uncle was on the ground, where the foreman couldn't see him and they let the mannequin fall beside where my uncle was. He came walking out of the trees rubbing his knees and told the foreman "if that happens one more time , i'm quitting." He said he thought the foreman was going to pass out, he asked my uncle "did i just see , what i thought i saw", my uncle said yea, it's happened twice today and i'm getting tired of it.
tell some one to get a bucket of steam, get a match to check for gas leaks, put blue dye on their tools (blue hands that don`t get clean)left or right wrench, fill work boots with grease,staple jacket to wall when left on hook,could have posted more. i have a book tilted spite mallious and revenge but i think it`s at work love messin with the new guy
Parking in front of the portapotty so the occupant cant get out is always a good one. My favorite is putting grease on the handles of the skid loader where it cant be seen.
Here's a few i've seen or heard about.

When welding forms the welder would start his arc so that a person holding the plate that had to be welded to the form got a little jolt.

Striking an arc on a person's exposed steel toe.

In a welding class the guys cut the lock on a guy's locker and everyone started spitting in his gloves, well he puts his hands in the gloves... you can guess how angry he was.

I once mooned a coworker to get him to shut up.

Another guy dropped his pants and tucked his junk between his legs then started running at his boss screaming "I'm a girl, uncle (insert name here)."

Guys taking and hiding company trucks is another one.
(dangerous) fill a brown paper bag with the acetaline from your torch and staple it shut, then toss it over to a guy on a grinder.

ask a guy to hold the spark wire on a mower and pull the cord.
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