Rebooted a production server when I thought I was on my personal PC because I couldn't figure out why my flash drive didn't appear when I plugged it in. (Stupid KVMs...

)
When in college and doing an assembler project at home, somehow nuked the partition table of my hard drive and corrupting the running DOS, preventing me from copying anything off. No idea what I did either, since the assembler program was stored on said drive...
Corrected a college instructor, in class, and in front of everybody. I was right - the flowchart of the program we were supposed to write wasn't efficient. My program, built the way I told him to do it, ran several orders of magnitude faster than everybody elses. He didn't appreciate it though, even though I was respectful.
Different instructor, whom I had almost no respect for, told us to turn off our monitors so he could lecture. "You can still work on your program, but your monitor has to be off." he said. Well, I clicked syntax checking off in QuickBasic, turned off my monitor, and kept coding.

He was royally POed because I wasn't listening to him blather on about crap I already knew (I knew more about QB than the instructor did by far), but I simply told him that I was only doing what he said we could do.
Spent an entire semester re-writing an instruction manual for an independent study in college. (Last required class and it wasn't offered otherwise.) Ran my PC without a UPS. Power went out *just* as WP/DOS was doing it's backup - corrupted both the original and the backup.
(QuickBasic, WP for DOS... I'm showing my age, aren't I?)
While modding my home PC, I took a molex off the power wire, sleeved the cables, and put it back on. Plugged everything in and couldn't figure out why my new DVD burner ($165 at the time) didn't work. Took my machine apart and made certain everything was seated well. Wondered about the odd smell, but didn't worry about it. Went back and played with windoze 95, trying to get it to work. Finally the smell got so bad I *had* to pay attention to it. Turns out that when I re-connected the molex, I transposed the 5 volt and 12 volt wires; the smell was the chips on the logic board of my DVD burner melting. (Literally, melting. Took it apart afterwards and looked like it'd been boiling on the logic board.)
Tried to wire something hot at work so 20 people wouldn't be idle. A wire nut in the bottom of 8" deep box fell off, exposing several wires that wouldn't reach up. Shut down all the machines, killed the circuit, tested everything with my multimeter to verify it was 100% dead. Reached in and started re-connecting the wires and got zapped. The 20 people on the other side of the room on a completely different circuit all went down and 15 of those needed new PSUs. (STILL can't figure out how the building is wired, nor how everything registered as 0 volts and I still got zapped or how changing something on one circuit that was turned off affected another circuit, so much so that it killed 75% of the power supplies in the machines on that circuit...)
Laughed out loud to the VP of operations at my last job. He wanted to buy a new compound bow for hunting that fall and was looking online at work. Suddenly he jumped up and literally ran out of the building. He was headed to the admin building to tell the guy who did nothing but monitor what people were doing online (yeah - they were that bad) that he did NOT intend to access what he did. FYI, for anybody interested, <4-letter censored shorthand for Richard starting with d, ending with k, with ic in the middle>s-dot-com is !!NOT!! <shorthand for Richard>s Sporting Goods...

I could not stop laughing about it and he tried to get my fired.