Dicks, death dust & denial.
I was asked to run some Cat5 into a workshop because the occupants, who'd previously resisted any attempt to put computers in there, decided they now couldn't live without one and why hadn't I done it already? This was obviously a 'do it by this afternoon or your boss will hear about it' sort of deal.
As usual I diligently checked the asbestos register and discovered that the dividing wall between the workshop and the main corridor (easiest run back to the local switch) was plastered in the stuff. Fortunately the wall of the room adjacent to the lab was clear, so I could just swerve the cable in earlier and approach from the side. Sorted.
I climbed my steps, pushed aside a ceiling tile and poked my head in. A quick sweep of the torch revealed something akin to to the surface of the moon, with dust and rocks everywhere.
You know those moments when your reptilian hindbrain is way ahead of you? It held my breath on my behalf, as I swung the torch around to see a foot wide hole smashed through the asbestos-infested wall. A contractor had run a dust extraction pipe with all the finesse of a rodeo bull in a mosh pit, leaving the entire ceiling void peppered in death dust. No cable for you, workshop guys.*
That mess was still being swept under the carpet when I left six months later.
* yes, I could have come up with alternative runs, but they were being dicks about the whole thing and telling me precisely where and how things needed to happen, so I took them at their word :)