It would entirely depend on the type of game you're playing.

If you're playing in any sort of tournament or other "serious" venue, you're SOL.

In a casual setting, it would depend on your opponent, I suppose. If they agree to rewind, then you're good.

But to put a proper answer in, strict RAW says that once you've done it, you've done it. 

Can a card be un-cast, if the player realises it was a bad decision?
I logged into the Ubuntu 12.04 LTS and instead of installing it... I pressed the Log Out button, thinking of installing it some other time.

But then a screen appeared asking for Username and Password.

I typed in my name as the User name and a password of my choice..I was halfway through typing my password..just then I remembered that I had to Login as 'root' so I hurriedly pressed the Back button... but instead the user name and half password I typed.. got submitted.

Now it is asking for User name and Password and when I type the ones which I had initially typed..its showing Login Error :(

But the problem doesn't end here... m unable to shut down my laptop.. its been more than 12 hours and m stuck at this login screen... plzzz help me out!
