It's the late 90s and I was working in IT for a small (200 employees) company.
One of my tasks was performing daily data backups, which ran for about four hours and started after everyone else had left for the day.
We had an intercom system in the building, where if you picked up your phone and dialed a specific extension, you could page someone, or make announcements so everyone could hear.

One night I was in the process of running Ethernet cable from switches on the second floor down to individual terminals on the first floor, so there was a lot walking around the building.

I like to have music when I'm working, but I didn't have an MP3 player at the time, so I thought... "I have music on my PC and no one is around. Why don't I just...", and I went around and cranked the volume knob on every ceiling intercom speaker in the whole building. Then I dialed into the intercom from the phone in my office, put the handset up as close as I could to my PC speakers, and boom, music throughout the building.

Cut to a few hours later, it's late, I'm tired, the backup is finished, so I close down the system, hit the lights, lock the doors, and go home.

The next day, I come in, not thinking about anything from the night before, and get to work. About 11:00am, the owner's secretary got on the intercom to page someone.... and blew everyone out of their seats. There was an almost collective, "What the fuck??!!" from everyone, and an "Ohhh, shiiit" from me, because of course I immediately knew what happened.

I ran around like a madman, turning the volume down on all of the speakers that I had forgotten the night before, while saying, "No idea... Yeah it's crazy... No no, it's fixed now..."

No one ever found out exactly why the volume was so loud that morning.