In life there are no takebacks. "Chess is life." (R. Fischer) Ergo, etc.
You say your mouse slipped? That's no different than a Fingerfehler in OTB chess. And if a man cannot control his mousie, is he a man or a mouse?
In an unrated game, I may allow a takeback, but I will never ask for one, and you have no right to one.
Actions have consequences. Take responsibility for the former; learn from the latter.
Be kind to your opponent; you need him to have a game. Don't whine if you lose. If you are paired with me then you are a patzer and can expect to lose many a game.
Play hard, but with detachment from the outcome. Apply this principle to your life as a whole. Don't blame others for your stupidity. If you are a snivelling crybaby who can't stand to lose, find another pastime, or else play your computer at its lowest setting.
Thus spoke the Sage of the Superstitions.
Favorite command: set busy 2 Second favorite: +censor [name of offending party]
All churlish rascals will be forthwith zapped into the outer precincts of cyber-obscurity there to languish with rest of their scrofulous ilk for all eternity.
If you wish to be removed from my censor list, I charge a nominal fee of 100 USD. Take advantage of this offer now before prices go up.