1. windows installing both programmers' and typists' Polish keyboard layouts by default + shortcut to change between them (I think it was alt+shift+space or even alt+space ?). Most people had no idea there even are 2 layouts and if they randomly pressed this shortcut and logged out - the next time they tried to log in - Z and Y are switched and if they had Z or Y in their password - they can't log in. This one caused millions of support calls.
2. corporate computers with English and Polish layout installed - if you happen to have layout switched to English and try to write an email in outlook - you will likely start with "cześć" ("hi") and when you press alt+s for "ś" it will send the "cze" as the full email. "cze" is very casual "hi" like writing a business email: "yo".
3. same as above but in eclipse with some version control plugins alt+s committed the code. Especially frustrating in cvs/svn.
4. python2 code often breaks if you have Polish letters embedded in code as string literals. It depends on the default system coding page vs default python coding page and other stuff, for some reason it's not utf-8 by default in python2. The solution is to use python3 or mess up with the default settings.