My favourite one of these has to be Outlook Online blocking nearly all Python, Bash and exe files sent via email...at the Faculty of computer science. Who would ever need to send code to others at the computer faculty? And it's not like it prevents you from sending it, it just prevents the recipient from downloading it, so there's no indication of anything wrong until it might be too late. Oh, and it doesn't even say "hey, that's malware". It just shows a little red X and removes the download button. No tooltip or popup, it just looks like a generic error. Had to go to Reddit to find out it's an antimalware thing.