Zac Efron had a mole on his left cheek removed after principal photography wrapped in October 2009. In May of 2010 he did a re-shoot of the scene where he shows Tess his new boat. The mole is present for the whole film, then disappears.
At 1:00:21 when Charlie tells Tess "I just can't," both his arms have been up, grasping poles, but when the camera changes, his right arm is down.
When Charlie is caught by Tess looking at her boat, the collar on his jacket is lying flat. Right before he says, "Flukes!" the collar is up, then down again.
The burn marks on Charlie's chest would not happen unless there was a flammable substance on his chest when he was defibrillated (and if this was the case you would have seen flames after defibrillation in the ambulance scene). Regular defibrillation does not leave any marks or scars.
When Charlie's EKG is 'flat-lining' (asystole), the paramedic defibrillates him. Defibrillation only works when there is pre-existing electrical activity in the heart, if the patient doesn't have any electrical activity ('flat-lining') resuscitation it achieved with drugs and CPR.
When Charlie and Sam are sliding on garbage cans in the forest during "mud games", the area they slide upon is too green, too well-defined, and obviously a "slip and slide" created with astroturf.
At the beginning of the movie (set in 2005) when they are showing a Redsox-Yankees game, they show Dustin Pedroia. Pedroia wasn't on the Sox until 2006
Charlie mispronounces the location of the Red Sox's AAA team in Pawtucket, RI, as "PAW-tuck-it." It is actually "paw-TUCK-it" and, since most locals pronounce it "puh-TUCK-it" - with the "aw" part barely vocalized - this is how most people associated with the Red Sox organization pronounce it as well.
In the graduation scenes, Charlie and the other students with the yellow honors stoles were wearing them backwards, the ends should be in front, not back.