I once spent a day debugging some data that came from an English doc written by someone in Japan that had been pasted into a system and caused problems. Turned out to be an en-dash issue that was basically invisible to the eye. No love for en-dash!

Similar.

Compiler error while working on some ObjC. Nothing obviously wrong. Copy-pasted the line, same thing on the copy. Typed it out again, no issue with the re-typed version. Put the error version and the ok version next to each other, apparently identical.

I ended up discovering I'd accidentally lent on the option key while pressing the "-"; Monospace font, Xcode, m-dash and minus looked identical.

This issue also exists with (so called) "smart" quotes.

Which, the iOS keyboard “helpfully” uses for you.

Pretty much the first thing I turn off on a new laptop (it's in the keyboard settings on iOS too.)

Especially when you're sending some quick scratch code in a slack message.