John Fiedler, Carole Shelley and Monica Evans are the only actors to appear in both The Odd Couple (1968) and The Odd Couple (1970). However, of the three, only Evans and Shelley reprised their roles, while Fiedler plays two different characters on two different episodes.
What are you doing?
Oscar Madison: Sterilizing the wound.
Felix Unger: With beer?
Oscar Madison: It's got alcohol in it.
The apartment that Felix and Oscar share changes completely from the first few episodes. Since they don't move and the first is clearly larger than the later one, it's an error of continuity.