Continuity mistake: Early in the show, Niles has his hand up on his tote bag talking about the mosquitoes at the cabin, but the last shot shows him with his arms down by his sides, with no time for the change.
Continuity mistake: Marty brings a six-pack of beer to the cabin, and puts it down on the table where it stays for the scene - but during the scene one of the cans is sticking out for a few shots, then returns to normal later. No one went near the beer during that time.
Answer: It equals the record of James Arness of "Gunsmoke" for the actor who's played the same part on TV NOT in a soap opera. If you include soaps there are two main contenders - William Roache, who's been playing Ken Barlow in "Coronation Street" in the UK since 1960. Don Hastings has been playing Robert Hughes in US soap "As the World Turns" since 1960 as well.