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The negative is electrically connected to the car's chassis. Nearly anything metal in the engine compartment has only a few ohms resonance to the negative terminal. If you are taking off the negative and your tool slips and you maintain contact between the negative terminal bolt and any metal part in the engine bay, then nothing happens. If your taking off the positive terminal and the same thing happens (you connect the positive terminal battery post to any metal part on the engine) one of two things can happen. 1) if the neg. terminal is still connected, you effectively made a short circuit since the engine is electrically connected to the neg terminal. This cause sparks, heat, and possible damage to the battery and your pride. 2) if the neg. terminal has already been disconnected, then nothing happens as the engine is not electrically connected to the neg battery terminal.
Listen to your dad
