A Malaphor, or an idiom blend, combining "hit the nail on the
head" (to do something exactly right) and "another nail in the coffin"(something that
will harm or destroy someone – e.g.,
cigarette) to mean someone succeeded at doing something that may lead to their demise or the demise of another or just that they did a great job at something that they probably shouldn't have done at all.
Best used sarcastically.