I have a Macbook pro with Yosemite OSX latest version. I've noticed a strange occurrence that seems to be related to Yosemite only because when I had Mavericks I never ran into this issue before.
After my laptop battery has died or it has gone to sleep I plug it into the power source and press the power button. The laptop boots up immediately to the login page. I enter in my password and then the screen goes grayish/blank and appears to be logging in. Then it seems to go into login limbo for a few seconds. When it is done 'logging in' the log in screen asks me to enter my password again to log in!
I have attached a video describing this. (sorry for shaky camera, but at least you will see it perform strangely)
processor is i7 quad core
5400 rpm hard drive
There are not any other issues with the hardware or software.
Also, sometimes when I am in this login screen loop it just powers off! So I have to hit the power button a million times until I get ANY response.
In any case, I eventually get to log in, but this is after 5 minutes of trying...which is annoying :(
Any help is appreciated.