Don't know why anyone's modded you down. I was thinking that either the interpreter, e.g. ruby, or the script itself already returned a non-zero exit status and the makefile detected that. That doesn't cause make to remove the target.

$ make foo

false >foo

make: *** [foo] Error 1

$ make foo

make: `foo' is up to date.

$

I suppose they could do "cmd foo >bar || rm bar" instead of the mv I suggested. But that causes no bar to exist whereas the mv leaves the pre-build one behind.