When Frankie Four Fingers is talking to Cousin Avi on the phone, Frankie is in a clothes shop and every time the picture turn to cousin avi then back to four fingers his clothes have changed. This happens about 3 times and his shirt even changes. It is impossible to change his clothes this fast.
I think this is what the director intended. This couldn't be a slip-up, not when the suits the guy's changing are so different and it all happening so fast. Anyone can see the clothes changing, it's more a joke than a slip-up...;]
Albalak is right, it was intended as a joke. Guy Ritchie was just messing about, it's all about humour (which explains Benicio Del Toro in his underwear...)