At work I'm running Ubuntu 14.04 and after it's been locked for too long, my first attempt to login/unlock the computer will be met with
Invalid password no matter how careful I am. The second attempt always works. If I am away from my desk for a couple minutes and come back, it also unlocks properly on the first attempt; I'm not sure how long it has to be locked. (Note that it is not requiring the correct password twice: I tried entering a wrong password first followed by the correct one, and that worked fine.)
I've found one or two posts of people reporting this on first boot where the login process would start and then kick them back to the login screen, but that's not my issue; the error shows immediately, and I have no problems logging in right after a fresh boot.
The closest match I found was this deleted question but I don't know if an answer was ever posted there since I can only see an old cached version.