Around Stardate 47569, Ensign DeCurtis was put in charge of checking the security measures surrounding the Paradan peace talks. Chief Miles O'Brien rebuked him for working on this without his permission, since he had discussed the security measures with the Paradans.
When O'Brien wanted to check the security of the quarters assigned to the Paradans, DeCurtis lied to keep O'Brien out. It was later found out that he was acting under orders of Commander Sisko, who was suspicious that O'Brien had been replaced by a replicant. (DS9: "Whispers")