What happened?
Well it caught me by surprise, but in hindsight it made sense. When making the mountain, I used a layer of cotton sheeting underneath the plaster. When I drilled through the plaster to plant the tree, the sharp drill bit caught the cotton cloth. The cloth then wrapped around the drill bit, pulling it towards the drill. This caused the thin layer of plaster to burst out and it sent bits of landscape flying. The drill also moved sideways as the cloth took a firm grip on the bit.
Here is what the hill looked like minutes before:
I had friends coming to visit the layout in 2 days so I needed to repair things right away.
I tucked the cloth back in.
Added plaster
When it was dry, I did some chipping at the surface, painted it black and then brown:
Added some grass back and tried to blend it as best I could with the original surface. I then planted the trees, but without drilling into cloth, now I make the holes with a bradawl.
This time I added a rough path down the slope.