>>212294484He wasn't actually admitting that he didn't put enough emphasis on the importance, he was trying to calm himself down because he needed to think clearly.
If Vincent hadn't been a heroin junky that was constantly spending large amounts of time in the bathroom from constipation Butch would probably have been killed when he went back to the apartment. Really if a whole bunch of crazy shit hadn't happened her forgetting the watch would have gotten Butch killed. Literally every event in the film positions all the other characters and their mentalities in such a way to save Butch from Fabienne's colossal fuck up.