Continuity mistake: As Harry is leaving the party he is dry despite having just been in the shower fully clothed.
Continuity mistake: When Ms. Dubcek is showing the Solomons the attic apartment in the pilot episode, they enter (and she later exits) through what is Dick's bedroom in all subsequent episodes. Additionally the staircase at the back of the living room that serves as an entry point is completely missing.
Chosen answer: Officer Don ended up appearing in 100 of 139 episodes, but I would say season 3 is when he become permanent as Wayne Knight's name appears in the opening credits. However, as he did appear throughout season 2 (first appearance S01E17), it's possible they already considered him permanent at that time since season 3's opening credits also included Simbi Khali (Nina) and Elmarie Wendel (Ms. Dubcek).
Bishop73