Funniest Practical jokes you've ever pulled

[Diofan]

What was your best Practical joke? Here's one of my many practical jokes:

In 1989 A friend of mine who was not "mechanically inclined" had a burnt out turn signal bulb. He asked me what he needed to fix it. I told him to go to the auto parts store and pick up some "blinker fluid". Picture this guy hitting every auto parts store in town looking for blinker fluid. He told me when he got back that he couldn't find any blinker fluid and every auto parts store clerk he came across laughed their asses off at him. (For those not mechanically inclined, there is no such thing as "blinker Fluid".)

[wavemeister]

Two practical jokes we pulled at the army:

The headlights of the Unimog truck are pressurized pneumatically to prevent water leaking into the case when driving through a river. A second line (with way more pressure) is running right beside the headlight line, ending in a coupling for trailers.
For instant fun, just depressurize the system, take a newbie, add a false information about connecting the lines (it doesn't matter that the lines are clearly different - order is order), start the engine, and...pop goes the headlight.
Of course, you can't pull this stunt on every occasion, but once a year, one headlight and one newbie had to be victims.

Another sure-fire way to create a laugh were the instruction drives for the handling of the Mercedes G in the field. You only need to rely on the natural stubbornness of the draftees and their readiness to disobey orders (even if they make sense).
Now look for an especially bumpy way, make sure that the backseat riders are disobeying the order to wear seatbelt and helmet and accelerate the jeep. After a short time, you should be able to notice the distinctive sounds of painful yelps when their heads are colliding with the roll bar.
Of course...don't accelerate too hard, and slow down if the yelps are turning into constant whinings.

[Diofan]

You could get in trouble in the US Army for messing with equipment like that...but still I can see those poor newbies trying to explain the mishaps.

Here's another one of mine:

My (then) next door neighbor had just gotten married. They had his prized early 70's Ford Gran Torino (Think Starsky & Hutch, this car is EXACTLY like that one.) Decked out in your usual "Just Married" decorations. Some friends and I decided to take it ne step further. While the Bride and Groom were in dancing and having a good ol time in their reception, we snuck out to the car, grabbed the floor jack out of my car, jacked the car up, All of us had to shove the boulder up under the bell housing at the rear axle. I then rewired the horn to the ignition coil. The rest is history!

Okay for those that don't know what would happen...here's the rundown. The boulder raised the drive wheels like an inch off the ground (Not high enough to notice at first.) So when he started the car and put it in gear, the car's wheels would turn but the car is stuck where it is. By rewiring the horn to the ignition, as soon as the engine is started, the horn sounds and the only way to turn off the horn is to turn off the engine.

[wavemeister]

You could get in trouble in the US Army for messing with equipment like that...but still I can see those poor newbies trying to explain the mishaps.

Noz only in the US Army...but we were not letting the "poor" newbies standing alone in the rain unless they had really deserved it. And if one of the draftees behaved like an ass, he was treated like one.

Another tales of the "fun with cars"-series:

After jacking up the drive axle (1984) and covering the vehicle in shaving foam and toiletpaper (1985), the classes of 1986 who were about to leave school had to thought of a prank which would top the previous events.
While we were hanging around in the cafeteria, one of us came up with the idea of placing on of the teachers cars in the hall. Another one added the brainwave of placing that car right between two piles of the hall. After taking some measures, our choice fell on the Ford Fiesta of our biology teacher - handy enough and perfectly sized.

First task was to make sure that we won't get caught in the act...so we had to do it while a) the headmaster was away, b) the teachers were busy teaching, c) the janitors had to to be engaged otherwise and d) no one else could caught as much as a whiff of our devilish plan.
a) was no problem, since short before handing out the papers, the headmaster of our town regulary met to discuss school things. b) was also no problem, since in the last week before the final goodbye, we "quitters" didn't had practically any lessons anymore. For c), we had to rely on the old "flooding in the toilets of the gym"-standard to divert the attention of the janitors. And d)...now, "Operation Fiesta" was top secret, of course.
With about 25 men and three planks, we went to the parking lot of the teachers (which was just a few metres from the entrance - what a luck for us), lifted up the car, carried it right into the hall and placed it between the piles. The whole operation didn't took longer than 10 minutes, then we had nothing else to do but washing our hands (in innocence and the restroom) and waiting for the bell.

"Operation Fiesta" was a great success, at least in the beginning...there was a big hello until the owner of the car entered the scene. Boy...she was absolutely furious, threatening to call the police and stuff. Under usual circumstances, we would had admitted the complicity of that prank, but with the threat of calling the cops, we just kept our mouths tight shut, only offering to set the car back again. And so we did, taking the utmost care (like we did when we had carried the car into the hall).
Two days later, the lot of us were called into a classroom by the headmaster, being confronted with an estimate of costs about 500 Euros (1,000 DM back then) for a dinged fender and some scratches. We refusted hotly to pay for damages we didn't had done. To our favour, we had a trustworthy witness in the form of one of our janitors who had watched the whole release action, testifying that the car wasn't damaged like being claimed when we had placed it back.

A few days later, we left school. Okay, an overall positive reaction would have been better...but we who were involved in the "Fiesta incident" still thinks that it was a cool prank.