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The winner's trap: stalemate
No move, no check, no win.
Even with a queen or rook to spare, there's one way to throw it all away: stalemate. If the lone king has no legal move while it is NOT in check, the game is an instant draw — and your whole advantage vanishes.
Look at the diagram: it's Black's move, the king is not in check, but every square is covered. White stalemated a totally won position. The cure is a habit: before each move when you're winning, ask 'after this, does my opponent still have a legal move?' Leave the king one escape square until the moment you actually mate.
You're up a rook. The enemy king has no legal move and is NOT in check. What happened?
Answer the question to keep going!