I downloaded the 1.6 pre-release, due to being impatient with the real release.

I logged in once, closed my laptop. Five minuts later, i would login again only to be greeted by credentials does not exist. I double checked as you would, still i get the same message.

Then i checked online both at Mojang.com and minecraft.net, both sites accepted my credentials.
I tried deleting all my local saves other than my saves.
I tried the old launcher and had no problem.
I also tried changing password and then loggin.

I hope this enlighten my issue/potential bug.

Edit:
I see that this post has been uploaded more than once while i was typing the bug. sorry for double posting