Continuity mistake: In season 1 (1957), Beaver is in grade 2 and Wally is 13 and in grade 8. Tony Dow was born in April 1945 and Jerry Mathers in June 1948. This means that Beaver/Jerry would have been 9 and Wally/Tony Dow 12, with a 3-year age gap. In the last (6th) season, Wally is a senior which indicates that he must have failed a grade. Meanwhile, Beaver is in 8th grade. Interestingly, the actors were also 3 years apart in age. At age 12 in 1957, Wally should have been in 6th/7th grade, which is more consistent with him being a senior in season 6.
Leave It To Beaver (1957)
1 continuity 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.