Other mistake: In season 1, we learn that Beaver is in 2nd grade-Wally in 8th. So by the end of 6 seasons, Wally would be in college. Tony Dow and Jerry Mathers are actually 2 years+/-apart.
Leave It To Beaver (1957)
1 other mistake in show generally - chronological order
Starring: Barbara Billingsley, Hugh Beaumont, Jerry Mathers, Tony Dow
Wally's Practical Joke - S6-E35
Visible crew/equipment: When Eddie Haskell comes over to scheme getting back at Lumpy, just as Eddie, Wally and Beaver are heading up the stairs, a mike boom is visible in the upper left corner.
Theodore "Beaver" Cleaver: You know something, Wally? I'd rather do nothin' with you than somethin' with anybody else.
Trivia: Phyllis Coates, who plays the new neighbor Mrs. Donaldson, played Lois Lane in the first season of the Superman TV series in the 1950s.