Things that make you go arrrrgh!
I hate seeing other people's badly designed software. It makes all of us in the business look like fools. Next door have been having problems with their computer. They have got some educational software which for some reason requires the user to log in (bad design). It uses non standard windows (bad design) and it does not have a proper install/remove program (bad design). When you type your password into the password window it shows it in clear text (bad design) and if you press the return key at the end of the password it doesn't do anything (bad design) but it *does* recognise the return character as part of the password (incredibly bad design). Until you figure out what is happening it looks as if you have forgotten your password.
This simple piece of incompetence has caused huge amounts of grief because it sometimes seemed to work and other times not (depending on whether the user acted on the reflex to press return to log in. I had to apologise on behalf of my entire profession at this point. And I felt quite bad about it. I still feel a bit cross writing this now.