I remember the 5 and 1/4 inch floppies with the little hole in the disk so the computer could synch up on the positioning of the data. You could borrow someone's most precious disk full of irreplacable data, then return it thumbtacked to their bulletin board. (The key was to carefully run the thumbtack through the hole in the disk - unless you really hated the guy and wanted to cause him unbearable pain.)
I also remember Z-100's with zdos. To format a floppy, you had to input "format a:". If you were absent minded and only typed in "format", your entire hard disk was formatted.
One of my coworkers was notorious for formatting our hard drive. Since we religiously backed up our files once a month, it wasn't disasterous, but still a major pain. I finally renamed the "format" command and wrote a basic program to format floppies. The program made it impossible to format the hard drive, but, being a smart aleck about our past problems, the program output funny messages about how the hard drive was being formatted no matter what you did. The guy that had the problem with formatting the hard drive thought it was hilarious.
At least, until the disk controller died all of the files had to be restored. Since it was a spur of the moment thing out of frustration, I never backed up the program that formatted floppies. The formatting fiend was showing a new guy around the office and told the new guy to type in "format". Being computer savvy (in fact, this happened to be the guy that sold me the dot matrix I loved so much), he kept telling the formatting fiend that he shouldn't do that. Formatting fiend kept insisting, reassuring the new guy that this would be funny as hell. The new guy finally typed in "format" and the computer started formatting without any of the funny messages that formatting fiend had become accustomed to. Finally, the formatting fiend realized what was happening and screamed in horror that the computer was formatting the hard drive. The new guy had to agree that typing in "format" was pretty funny, at that - in fact, this seemed like the most entertaining office he'd ever worked in.