Continuity

Continuity was not something early sixties television writers seemed very concerned with and I guess that's why there is so much inconsistency regarding Andy and Barney's relationship (are they friends or cousins?).

I have noticed that there are a few episodes that make reference to or tie into a previous episode. In season fives 'The Lucky Letter' Andy talks about the Gypsy fortune telling cards and lamp that Barney bought at auction and used in the episode 'Three Wishes for Opie'. At the end of the season four episode 'Andy's Vacation' Andy convinces Barney that he should take a little vacation and go to the YMCA. The beginning of the next episode 'Andy Saves Gomer' has Andy reading a letter from Barney in which he says he is having a great time vacationing at the YMCA.

Are there any other episodes that tie in or make mention of a previous episode?