After Rick sprays paint thinner around Frank's workshop, he ends up setting his right hand/arm on fire. When he leaves he opens the door with his right hand and holds up his left arm (as if that was the arm in pain). Once he exits, his right arm again is the one in pain.
During Susan and Danny's escape attempt, Danny starts to put on his jacket and has his backpack in his hand. In the next shot, his jacket is completely on and zippered without any sign of the backpack.
When Frank uses his cell phone to call their house near the end to warn Susan and Danny to get out, Rick checks the Caller ID, which comes up Frank Morrison, but with a "410" area code, which is a Maryland number. The movie is set in the area of Southport, North Carolina, which would give Frank a "910" area code. Coincidentally, at the time of filming (prior to number portability) all cell phone numbers in Maryland used the "443" area code.
When Frank and Rick are fighting in the garage, Rick knocks Frank to the floor with a paint can. The light shines onto Franks face, when Rick picks Frank up off of the floor, revealing an obvious stunt double.
The goof items below may give away important plot points.
After Rick Kills Ray, he pulls up to the Brick Works Company, gets out of his truck and slides open the wooden door to enter the Brick Works building. He drives the truck through the open door and parks inside the building. After sliding Rays body into the furnace he opens the sliding door again, the door that he never closed.
At the end of the movie, when Frank, Rick and Danny are sailing Frank gives direction to turn the ship around and starts doing a tack (shifting the sailboat with the nose into the wind). Checking the horizon we can see the boat turning right. The next shot (taken from above) shows the boat making a jibe (shifting the sailboat with the stern through the wind), turning left.