Season Three: "Running With Scissors"
Carrie loses her mind and cheats on Aidan with Big. This is the episode she gets caught by Big's wife, Natasha — in Big and Natasha's apartment, no less. Natasha falls down stairs, and Carrie has to take her to the ER. It's a terrible and dramatic ending to Big and Carrie's affair made worse by the sweet Aidan waiting for Carrie back at her apartment, completely unaware of what his girlfriend has done. Grrr.