When the band starts playing in Mr. Wilson's house they are standing very close to Mr. Pindyck but in the next shot showing Mr. Wilson and Mr. Pindyck sitting together they are nowhere near him.
Mr. Wilson's sign to promote the Bird Society meeting reads "To day 4:P.M." instead of "Today 4.P.M."
When Mrs. Wilson opens the kitchen door to let Dennis and Tommy in with the groceries, the door opens outward instead of inward.
The Mitchell and the Wilson homes are side by side and share a backyard fence with a gate between them. When Mr. Wilson gives Dennis the film for Henry to develop, Dennis runs toward where the gate is, but then appears running toward the front door of the Mitchell's house, which is in the opposite direction of the gate, when Tommy stops him.