Smallville Season 10 Episode 15 (02/25/11)
After preparing to celebrate Lois and Clark's impending nuptials in separate guys/gals nights out, Clark and Chloe wake up to the hazy revelation they have have gotten married.
As they tried to piece together what happened, they discovered Zatanna had a sent them a bottle of magically spiked champagne. They tracked Emil and Tess to the Ace of Clubs where they were singing karaoke. Emil gets arrested for a disappearing armored car, which Clark discovers crashed into the barn at the farm.
Lois and Oliver woke up in a bad neighborhood on train tracks, also missing memories of the night before, as well as Lois' engagement ring. They discovered she lost it at a casino. They are both kidnapped by the criminals searching for the lost van, but escape in showgirl disguises.
Clark and Chloe crash the party and help rescue Oliver, Lois and Emil.
Chloe decides she can no longer stick around and be Watchtower, so she leaves for good, but not before Oliver tells her they got married.