This is a wonderful congruent conflation of “straw that broke the camel’s back” and “the final nail in the coffin”, both meaning an event or action that ultimately leads to the failure of a situation. This malaphor was heard by Elizabeth Poulsen, the daughter of frequent malaphor contributor Marcia Riefer Johnston. Marcia indicated that Google brings up many instances of this phrase, which made her wonder if it is an intentional mashup. After looking at a few of them, they all seemed to be unintentional to me, making it a legitimate malaphor and an excellent one as well as it is congruent. One example found on the internet is a comment noted in the Hollywood Reporter discussing the women in Arrested Development:
Having a crush is normal, especially for costars, but Bialik has taken her infatuation with Jim Parsons to a whole new creepy level that seems to make even Parsons uncomfortable sometimes. She’s obsessed about talking about him, and manages to push her way beside him in press photos all the time, it’s unsettling. I’m not saying this is the nail that broke the camel’s back in her marriage, but if I were her hubby, I wouldn’t stand for all the pathetic Jim Parsons fawning from my wife either. Just sayin’.
Thanks to Elizabeth for sending this one in!