I am, just like any other bike mechanic, prone to practical jokes. We all have removed each other's cranks and reinstalled them at 90 degrees, turned someone's bars around while they weren't looking, etc.
Possibly the meanest thing I ever did though, was to build an unexplained click into Scott's wheels.
Scott had ordered in some Hugi hubs and Mavic rims to build up a lightweight wheelset. In order to make it as light as possible, he also got titanium spokes. They were all the rage, that year, despite being somewhat flexy and hard to tension.
Scott asked me to build the wheels up, for him, and I did (even though I didn't want to). The ti spokes were, as I said, hard to get evenly tensioned, and each wheel took way longer than it should have. I mentioned this to Scott, as I was tensioning the first wheel, and he basically told me to quit my bellyaching and hurry up with his wheels.
Well, he was the boss.
So, on each wheel, I dropped a spoke nipple into the hollow rim. They had single eyelets, in order to make them lighter, so the hollow inside was accessible from the spoke hole on the tube side of the rim.
I then got the wheels built, and tensioned, mounted the tubes and tires and installed them onto Scott's bike. By this time, it was past quitting time, and Scott had gone home. So, I took them for a little test ride in the parking lot.
Sure enough, if you were riding along, the wheels were dead silent. But, at a walking pace (or hill-climb pace on a lot of trails), the loose nipples inside the rims would slide back and forth between two spokes, hitting the spoke nipples inside each rim.
It was not loud, but just loud enough to hear it. They only moved twice per revolution of the wheel, so it was hard to pinpoint where the noise was coming from.
The next day, Scott took the bike for a test ride. As he walked it back into the shop, I saw him listening intently to the bike. He would take a couple of steps, then cock his ear toward the bike.
He never did ask me what was making the noise, even though he would comment on it, occasionally as he rode slowly up the trail, or walked across the parking lot.
About 10 years later, I finally told him what I had done. I figured that enough time had passed that he would find the prank amusing.
Plus, I no longer worked for him, and he couldn't fire me!