Roof Ladder

Here's a nice little anecdote from one of my previous lives, my telephone engineer phase.
The job involved the installation of a payphone and telephone line into one of Oundle School's boys boarding houses. This was a two man job due to the size and age of the building (100s of years old with no modern infrastructure) and the necessity for multiple overhead wire spans to get the line into the building.
The internal work is completed without drama, payphone installed, internal wiring completed. I cannot recall if it was the case here, but previous boys houses had their payphone bells disconnected and mains powered flashing lights used instead, "to avoid disturbing the boys during prayer".
The external wiring is a bit more physical, thankfully there are poles erected to carry wires from the rear of this building round to the front and the road. This initially goes fine, some clambering about with the dropwire dispenser between poles, but the spans are going up ok. Then the sticking point.
Two spans left to go, pole to a chimney on a bungalow, from the chimney to the final pole and connection to the outside world. Ah, a chimney. Other wires are already attached and we can share of one of their brackets. But, how to get up on the roof to it? This needs a roof ladder, which we don't carry as routine. This isn't good, 45 minutes back to base, paperwork, and return. Is there another way?
Well, we only need to be up there for a couple of minutes. Attach clamps, tension and clamp first span, clamp second span, and down. Hmm... this isn't a tall bungalow, I can reach up and touch the rain guttering. Ok, a plan. We extend our 3 section ladder to 2 sections. Stand it at the bungalow front, lean it against the roof edge, with one section now pointing into the air. I will climb the ladder, as it pivots the bottom will lift and my partner (the heavier of the two of us) will grab the end and take the weight of the ladder and me. I will climb up the now flat on the roof ladder with clamps and wire between my teeth, at the top I will sit astride the roof and do the job. Easy.
Not quite so easy, here's what happened... I climb, the ladder pivots on the roof edge. My partner grabs the ladder, I start to climb. He yells out, "I can't hold it...!" The ladder slides down the roof, I do the cartoon-like scrabbling my way up the ladder without making any headway. As the ladder disappears beneath me I throw myself up and forwards onto the roof. This could still work, I am now on the roof, just need to clamber up...
Nope, the roof is old. Wooden tiles, covered in moss, slippery moss. I am sliding backwards, no grip, fingernails scraping their way down. I stop with my legs hanging over the edge, not high up, only 8 feet or so. Looking down I assess the situation and politely ask my partner to get out from lying under the ladder and put it back up for me.
After a hearty laugh we consider our options. The proper roof ladder will have to be brought to the site, no doubt about that. But first, hmm... I seem to have kicked down a huge section of this innocent customers rain guttering. We spend the rest of the day fixing the guttering, nobody was home, we did a good job.
The job is finished the next day, with an official roof ladder. And yes, I did get my current job based on my excellent health and safety knowledge...