Dax invites her close friends(And Leeta) to join her in an important Trill ceremony that let's her communicate with her previous hosts. But things hit a snag when Curzon decides he doesn't want to go back.
Apologizes a Lot: Tobin can't go two sentences without apologizing for something. He even apologizes for apologizing.
Body Surf: The Guardian allows the personalities of Dax's previous hosts to move into different bodies.
Didn't See That Coming: Invoked. For the stress test, Nog had been practicing in the shuttle. Then O'Brien brings up the test and he's in Ops with a different problem than he practiced for. It wouldn't test his response to stress if they gave him what he prepared for.
Retcon: In the Next Generation episode The Royale, Captain Picard mentions that Fermat's Last Theorem has still not been proven by the 24th century. A few years after that episode was aired, Andrew Wiles was able to prove it. This episode acknowledges Wiles' proof, though it also tries handwave Picard's comment by suggesting that mathematicians are still trying to find a better way of proving the theorem.