Why does *[[2 3] [0 x]] crash if x is a cell? Well no other rule but line 17 applies.
Why does *[[2 3] [0 x]] crash if x is, say 4?
/[4 [2 3]] =>
/[2 /[2 [2 3]] =>
/[2 2]
which crashes because, at that point, no other rule but line 17 applies.