The one where OneDrive moves all your data to the cloud without asking, then warns you you’re running out of cloud storage space, and when you then disable it, it deletes your stuff from the cloud without moving it back to your disk.
That one’s good fun for the whole family!

Or the one where OneDrive in its default config uploads your Documents folder to the cloud.
And Outlook 2019 in its default configuration stores the .pst file containing your e-mails in the Documents folder.
And Outlook .pst files get corrupted when you access them from inside OneDrive.
So a default MS Office install can corrupt your e-mails if you click “yes” on everything Microsoft “recommends”.

99-pedes, they are off by one bugs.

“We can’t send e-mails farther than 500 miles”

OpenOffice does not print on Tuesdays

Race conditions

Daddy longlegs. They keep the garage clean of other critters and spiders.

It’s more of a personal one.

I made my first touch game in Unity. I tested all the system and everything worked after weeks of testing and I wasn’t able to find more bugs.

I gave my phone to my wife and asked her to try it. Immediately she was able to get the character to warp across the screen. I didn’t code this and based on my code this was impossible to happen.

After more testing I discovered that it wasn’t my code which was bugged but Unity’s. Since she was touching the screen in a way that it didn’t register that her finger was removed from the screen.

Ever since all my games touch inputs have manual timers to ensure that a touch is actually registered. And I give her my games to test since she usually breaks it in weird and wonderful ways.

Spectre, because it forces us to acknowledge the abstractions we build upon.

My favourite ones are compiler optimisations based on impossibility of Undefined Behaviour, like this one: https://stackoverflow.com/a/78751225/1122720

And time travel, of course: https://devblogs.microsoft.com/oldnewthing/20140627-00/?p=633

Bumblebutt, bumblebumblebumblebutt!