Is the equipment you’re about to work on really de-energized? How do you know? Did you flip a breaker and the light went off? Maybe you looked at the electrical drawings and verified with your DMM that the supply breaker is off. But this doesn’t necessarily mean the equipment is de-energized.
The drawing might not be complete or current. For example, undocumented cross-wiring or backfeeds can energize a circuit whose supply breaker is off. So can batteries and capacitors