Corrected entry: In the final scene after Matt Damon gets shot and falls to the floor and Mark Wahlberg walks out of the room, the camera shows Matt Damon lying in a pool of blood. A second later the camera cuts to a different angle of the body lying on the floor and it is no longer Matt Damon. It is a different actor with darker hair, darker eyebrows and wearing different clothes. (02:24:05)
Corrected entry: When Costigan leaves the adult theater and follows Sullivan to get an ID, his phone rings in the alley and this lets Sullivan know he is being followed. This seems convenient to the plot, seeing as how Costigan's phone was just on vibrate in the theater. (01:24:45)
Correction: Unless the keypad is locked, on most phones it quite easy to accidentally jostle it or depress buttons in such a way as to change the ringer setting. It happens all the time to me, particularly if the phone is in my pocket. Yes, it's a plot device, but an intentional one that serves to underscore the peril's of Costigan's double life.
Correction: It's still Matt Damon, you can tell from his "V" hair line, and he is wearing the same green shirt.