A mouse slip is an accidental online move, where the piece lands on a square you did not intend, and the move stands just as if you had meant it.
Online, releasing a piece on a legal square commits the move. Drop your queen one square short, castle by dumping the king on the wrong square, or promote to the wrong piece, and the position updates instantly. It is the online cousin of the touch move rule: intention does not matter, the move does.
Mouse slips cluster where speed does: blitz and bullet scrambles, hasty premoves, and drag and drop moves across a long diagonal. Lag makes everything worse, because the board can shift under your cursor between click and release.
You can engineer most of them away. Turn on move confirmation in slower games, prefer two clean clicks to one sweeping drag if slips keep happening, and slow your hand for one beat before releasing a piece in a scramble.
Only if your opponent agrees. Some sites let you send a takeback request in casual games, but nobody is obliged to accept, and in rated play you should assume every released move is final.
Enable move confirmation in daily and rapid games, use two clicks instead of long drags, and build the habit of pausing a beat before you release the piece. Cleaner mouse habits are worth real rating points in fast chess.
BetterChess is a practice tool — we make no guarantee you'll reach 1800 or any rating. Definitions are standard chess terminology; every diagram position is checked legal with the same engine the board runs.