When the Ricardos and the Mertzes are in the Ricardo apartment and are getting ready to leave for dinner, Fred tells Ricky that he called the plumber and he'd be up this afternoon to check everything over. It's already evening.
As the 2 couples are leaving the Ricardo apartment to go to dinner, the door says "3B", but the Ricardos live in apartment "3D"
At the end, when Lucy and Ethel go to see the men, who are behind bars, Lucy first kisses Ricky, then Ethel steps up to kiss Fred. Their lips/faces very clearly never get close enough to actually kiss. It's well-known that the two actors, Vivian Vance and William Frawley, couldn't stand each other.
The stack of plates Lucy drops on the floor are very clearly all attached to one another based on how the stack moves and falls.
The Ricardos and Mertzes are getting ready to leave for dinner and it is night time. Fred tells Ricky that he called the plumber and he's going to come up this afternoon to check everything over. Since it's already night time, the plumber would not be coming that afternoon.