Rule 5 of 40 · Chapter I — Before You Touch Anything
Never trust a switch alone
Why this rule exists
A switch, breaker, or relay is a mechanical device, and mechanical devices fail. Contacts weld closed, linkages break so the handle moves but the contacts do not, and breakers can read tripped while still passing current. A switch also only opens the conductors it is wired to; a shared neutral or a backfed circuit can stay live with the switch off. The switch is a control, not a guarantee. It reduces the odds power is present; it does not prove power is absent. Only a verified test does that.
In practice
Use the switch or breaker to de-energize, then lock it, then verify dead with a tester at the conductors. Treat the load side of any open switch as potentially live until proven otherwise. Be especially wary of three-way and four-way switching, multiwire branch circuits sharing a neutral, and anything that might be fed from two directions. If the labeling and the tester disagree, believe the tester. Never let flipped the breaker stand in for measured zero volts.
When it doesn't apply
None on the principle. A listed disconnect with a visible break and lockout provision is more trustworthy than a snap switch, but even then you verify. The only thing that proves a conductor dead is a proven tester reading zero at that conductor. If you cannot get that reading, treat it as live.