Deliberate "mistake": When Sullivan makes the phone call to Costigan using Queenan's cell phone, he leaves Queenan's blood on the talk button. It's been hours or even days since Queenan's murder, how was the blood still wet?

Other: In the scene where Costigan drags a handcuffed Sullivan into the elevator, Costigan slams Sullivan against the wall and his left arm is visible and not in handcuffs.

Continuity: At Billy's mother's funeral, the signature of Frank Costello on the flowers changes between shots.

Continuity: When Costigan is with Madolyn in her office, the tissues in the tissue box change consistently between shots.

Continuity: When Ellerby attacks the guy for not installing the cameras in the right place, he knocks off a map from the board. A second later Ellerby is still going after him, and the map is fixed.

Revealing: ***SPOILER*** When Costello is killed, a blood-pack is noticeable under his shirt. Submitted by Sacha

Continuity: In the scene after Queenan dies, when Sullivan picks up his phone to contact Costigan, he presses the "talk" button and a smudge of blood is left. In subsequent shots, there's no trace of blood on the "talk" button.

Revealing: When Costello's hooligans drive away from 344 Wash after the shoot-out, Queenan's body on the ground is quite obviously a dummy.