Depends on what you mean by "misused".

If the card was cast illegally, such as trying to Shock a creature with Hexproof, the game is rewound to the point immediately before the spell was cast. This does depend on how much time has passed- if so much time has passed that fixing it would be difficult, just correct anything currently illegal and continue the game.

If a player just did something silly, like trying to Shock a creature that actually had 3 toughness because of a Glorious Anthem, the answer is no in any tournament setting. Once you've taken an action, you can't suddenly change your mind. In casual play, it's up to the opponent. If the opponent decides it's sporting to allow take-backs, they can allow them.

Can a card be un-cast, if the player realises it was a bad decision?
So I started up the demo and it's asking me whether I'm a Spacer, Colonist, or Earthborn and whether I'm Ruthless, a War Hero, or a Sole Survivor.  Based on this forum thread it appears these choices didn't matter in ME1, it was barely mentioned, which seems strange given everything I've heard about your choices impacting the game.  Does it matter more in ME3?  Does it limit what you can do or significantly impact any character's dialog or attitude towards you?
