I think most of the stuff I make falls into this category...
My favourites are probably these two:
- a tool I made called gif-to-ansi (https://dom111.github.io/gif-to-ansi/) which converts an animated GIF to a stream of ANSI escapes in a shell script.
I made this because I wanted to loop a GIF image in terminal when I mistyped a command to train me to do better typing.
- a 26 "language" polyglot (https://codegolf.stackexchange.com/q/209669/9365) where each starts with a different letter of the alphabet and prints out the alphabet less it's starting char (e.g. Bash prints out ACDEF...).
This was just a puzzle that nerd sniped me, I really enjoyed it though and even got to learn a little APL for it, as well as brush up on shells. I highly recommend using sites like this if you want to explore a languages depths, I used it to learn Perl for a previous job and still play around in Perl today!
Years ago, on an old (~2000/2001, AMD K series maybe?) system, I was doing some patches for a friend of mine and noticed there was an update for his BIOS. Went through the normal process of backing up to a floppy (luckily, I didn't always do this on my own upgrades before this point!), began the flash process and it all appeared to work fine. Rebooted and the machine didn't come up at all. Blank screen, nothing. I totally panicked. He came to check in and I must've been profusely sweating by this point and we chatted it through, but I noticed, on reboot, the floppy drive gave a tiny click, not like the normal exaggerated one it would do when normally booting. I had a glimmer of hope that it might still read a boot record from the disk, which it did. When the floppy stopped grinding, I carefully typed in the commands (even though there was no output to screen), I knew were on the disk (after running back home to build a bootable disk with a flasher and the BIOS dump) managed to restore the BIOS back to running perfectly with the old version!
Was absolutely terrifying, but very rewarding to get back a machine that we were both sure was bricked!
I might be misremembering but isn't there a key combo ([Fn] + [Ctrl] + [F7] or something?) that enabled control selection and shows the underlines. I haven't used Mac OS for a while so I might be wrong!