Websites that have a password length limit that's way too short and/or don't allow symbols.
Example, I think EA has a password limit to 15 characters and doesn't allow spaces, underscores or even symbols, meaning that your password must be only letters and numbers, which is quite unsecure.

When I forget to put a small project I'm working on onto my USB or the cloud so I can't access it when I'm on the go. :'(