Mechanics and automotive technicians put in long, hard days and sometimes have to have a little fun, sometimes at others’ expense. 
I’ve worked at car dealers for a number of years and I saw my share of funny pranks, but hadn’t seen this one. I thought this one was pretty good. 
What pranks or shenanigans have you seen while performing car or truck servicing?
CSA
Back in the '90s, I was driving my mechanic around in my Taurus so that he could listen to a noise that had mystified me. As we drove around, my automatic door locks kept locking and unlocking–which was a new problem.
It turned out that he had rested his right knee against the door lock button on the passenger door.
He claimed that it was accidental, but I think that he may have been pranking me.
Mechanic did service on a vehicle and some other repairs to the transmission. The guy got done and backed the car out of the hoist and drove off for a test drive my boss took some transmission fluid and poured it on the floor mimicking the path of the car backing out. When the guy got back and saw the mess, there was some colorful language. It got more colorful when he found out what happened.
i saw this yesterday, and literally laughed out loud at it.
a while back, I was up on a ladder changing fluorescent light bulbs in one fixture in the shop, and whenever I would put in a new light bulb, the whole thing would go out. It took me a couple minutes to figure out one of my buddies was watching from the other side of the shop and flicking the other light switch on and off. 
Well, this wasn’t a deliberate prank by me but I did find it funny. Many years ago I had a city tractor-trailer and had a assigned tractor. It was a single rear axle Ford C-850 with a 534 cu in gas engine. It was getting pretty old and the air lines to the front wheel were plugging up with crud from a worn out compressor.
This made the front brakes not work and they were very important because much of my work was spotting or picking up trailers at customers and I had to drive one way with no trailer. All the we=ight was over the front wheels of this cabover and only the back axle had brakes. This was very dangerous and I kept writing it up and refusing to drive it.
One day I came into work and I was told it was fixed . We had a very large terminal and I drove it up and down the yard and it was obvious to me, the fronts were still no working so I wrote it up again and when the shop manager insisted it was fixed, I refused to drive it.
The shop manager came outside the shop door to observe and the shop foreman was instructed to come down the yard about 50 mph and slam on the brakes hard enough to leave skid marks from the front wheels. He did, and he slammed the brakes hard enough to unplug ONE front wheel which made the tractor spin to the left directly at the shop manager who had to dive into the very greasy dumpster he had been standing in front of just before the tractor slammed into it.
I just looked at the manager in the dumpster and the foreman in the wrecked truck and said, I’m guessing you want me to take a different truck.