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So you opened the breaker for this circuit and now it’s safe to work on. Right? Probably. But do you want to take the chance that:
Your normal sequence of steps to de-energize something will nearly always result in a circuit that is actually de-energized. The problem is with the word “nearly.” Several issues contribute to some uncertainty about the de-energization, and it takes only one contact with lethal voltage to make certain you’re dead. You won’t get to go back and solve the problem.
You need to solve the problem before the lethal shock kills you (actually, so it doesn’t kill you). And to solve the problem, you need to know it exists.
Thus, always test the circuit for power before proceeding as if there is no power. Because there very well might be power.
Verify using the right test equipment. For several reasons, a small “pocket” probe may indicate a safe situation that does not actually exist. So use a DMM.
But follow this procedure:
Re-check the meter against that known voltage.