It's incredible what you find years after the last contractor left. Another favorite of mine was an outlet labeled "DO NOT UNPLUG" in the rack room. It was connected to a jbox in the next room with some BX. An IEC cable was spliced onto another IEC cable (both ends were male) and attached to the PDU. So if you unplugged it you had exposed AC power.

And don't get me started on hvac guys and plumbers butchering joists.

We called those "structural ducts"