When Wally walks to the sink there is milk in his glass. When he gets there it's gone.
When Larry and Beaver come into his sister's room to read her diary, Larry's zipper is partially down...wardrobe malfunction.
There is a scene where Wally comes in through the front door, and Ward tells him that Larry Mondello has run away. Look at the door closely. It is not completely closed after Wally shuts it. After his dialogue with Ward, there's a shot of the door completely closed even though there was no one else in the room to close the door.
All of the upstairs windows in the Cleavers' new home have window boxes (with flowers) and they are flush with the window sills. However, when Wally drops the rope from his & Beaver's bedroom window down to Larry (to tie on his suitcase), there is no window box.
Beaver and Larry managed to smuggle 115-pound Larry upstairs without issue, yet inexplicably didn't bring his suitcase. That led to the boys covertly pulling the suitcase up past the den window, leading to Larry's discovery by their parents.