a couple of decades ago, I worked in the top floor of this old building in glasgow https://www.google.com/maps/@55.8601694,-4.2542338,15a,24.1y... . Ground floor of the building was a TGI fridays. The evac route was up onto the roof, across steel walkways to another corner of the building, and down a back stairwell.

There was a regular alarm test, so we knew where to go. Then one day there was a REAL alarm, we all trotted off in our designated directions, but as we came down that back stairwell, it was full of smoke (not thick, but clearly something going on...) then at the bottom we saw why: TGIF had been stacking cardboard waste in there, plus their employees had been taking cigarette breaks in there, beside the no smoking sign... and you'll never guess what happened. Yep, fire in the fire escape.

The fire marshall was getting very stabby pointy fingering at some sheepish looking people as we went by to our muster points.

(it was just a stack of cardboard and there was nothing else flammable in the stairwell, it wasn't life threatening, just reckless)