Can a card be un-cast, if the player realises it was a bad decision?
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.