Many online services have a little "Remember Me" or "Remember My Login" checkbox under their login forms. It's nice and convenient for frequent users that are on private computers.
For some reason though, many "new" sites seem to default to having this box already checked, and I always wonder what sort of brain dead person decided that was a good idea.
I forget the last site I saw this on, but it was within the last month. I had to explain the LOGIC FAIL behind pre-checking this box before, so there may be some people that don't see why this is an issue. I'll lay it out real quick – if a user wants to stay logged in to a service they can just click that box once. If a user doesn't want to stay logged in (shared computer, paranoid, many reasons why) they have to uncheck this box EVERY time they come to the site – and if they forget to log out when they're done, the next person on the computer is going to be logged in as them!
So, tell your web-app developery friends: "Remember Me" should NEVER be the default!