Continuity mistake: The scratches on Phil's neck have completely disappeared by the time they're back for the wedding.
Continuity mistake: The boys try to drug Mike Tyson's tiger by putting roofies into a piece of steak. When Alan puts the pills into the steak you can see that the meat is extremely soft; he pushes the pills in easily with one finger. When Stu brings the steak into the bathroom it's clearly a stiff piece of steak and not the same one.
Visible crew/equipment: Boom mic visible in the window on the right-hand side when walking down the hall way of the hospital.
Continuity mistake: In the regular, non-extended version of the movie, when the guys are on the roof of Caesar's Palace they drink a shot. Then without filling the glasses they have another toast with a suddenly full shot glass. [The extended cut shows them refilling the glasses, but it makes for a mistake in the regular version as there's no other indication of any time passing - the glasses are just suddenly magically refilled].
Continuity mistake: The morning after scene opens with Stu laying on the floor of the hotel suite. There is a chicken about 2 to 3 feet to the left of Stu about even with where his head is on the floor, but when they cut to another camera angle showing Stu getting up the chicken is now 2 to 3 feet directly in front of him rather than 2 to 3 feet to his left as previously shown.
Continuity mistake: In the beginning when Phil calls Tracy to explain they cannot find Doug, the three guys in the background are in a very different position than at the end of the movie during the same time period. In the end the drug dealer walks forward towards Stu and Alan and then Stu begins running over to Phil. In the beginning all of the guys in the background are farther apart during the same time of Phil's phone call.
Other mistake: In the morning after scene, when the three men are sitting at the dining table by the pool with the baby, watch the extras milling about behind them. The same two people pass behind them twice from the same direction just moments apart. A little later a woman in a swimsuit walks by behind two of the characters. After a few more lines of dialogue, she is still walking by behind another of the characters.
Continuity mistake: Once the naked guy is let out of the trunk and runs off, Alan is sitting on the ground with his back resting against the front tire in one shot, then against the car just behind the wheel well in another, then resting against the tire yet again in another.
Continuity mistake: In the opening scene when Phil calls Traci, Drugdealer Doug is urinating in the background for almost the entire scene. Later, when the scene is playing out again, Drugdealer Doug zips up and walks back to the car after Phil says "We f***ed up," which is the second line of the conversation.