I had a very similar experience when I was breeding mice for a large groups of snakes I owned. I used deep aquariums with 1/2"~12mm mesh tops and had shelves set up to fit. Kind of a 1970's rodent rack. The tops were just a gesture, the tanks were too deep for the mice to jump out.
I kept these tanks in a garage with old overhead doors that didn't actually seal, but did help mitigate temperature extremes.
On morning, I went to service the mice and found a juvenile Black Rat Snake (24"~61cm) had replaced a group of half-grown mice. He had gone through the mesh on an empty stomach, but could not get out after cleaning up the mice. He had tried to get out and got stuck in the mesh, and I had to clip the wire to extract him.