Mike says Paul was his Best Man when he married Gloria. Mike And Gloria's Wedding shows Mike's Uncle Kas serving as Best Man, and Paul is nowhere to be seen.
When Archie begins suffering his second toe spasm, it's suddenly his opposite foot that's bothering him (left one the first time, right one the second).
When the family is waiting for Paul's friend Jeff to come to the house to pick him up, after Paul refuses Archie's ten dollars for a hotel, you hear the doorbell ring off-cue. The cast ignores it, and then about a minute later after some more conversation, the doorbell rings at the intended time and it's Jeff.
In the final scene, the boom is seen above Archie after the other three characters finish singing.