Can't log in to guest account I can't log in to the guest account. Clicking "guest" at the login screen has it shake its head. Normally there's no password. What could be causing this? 

It might not be relevant, but it's a laptop with no battery.
In the Parental Control settings for the Guest User, make sure that all time limits are either unchecked or you are logging in outside one of the defined time windows.  If using Bedtime, also make sure your system clock is set to the correct time.