$_ is clobbered by debug trap

[HaleTom]

bats suffers from the same bug resolved in:

• Fix Issue #367: Value of $_ overwritten in bash postmodern/chruby#381
• Don't clobber $_ rcaloras/bash-preexec#32

This is a "feature" of bash, see this answer on SO.

I discovered this when my bash-preexec returned a false-positive due to this "feature" not being worked-around in bats.