In the beginning during the Carrie spoof, when the shot is wide, the black keys are on the left side of Stan's tie (right from our POV). But when they go into the close-up of Stan, the keys are on the right side of the tie.
After Stan knocks the dish and strawberries on the floor at the motel, the previously empty table now has a telephone, a drink and a napkin on it where there wasn't any before.
Steve's feet disappear and reappear during the water balloon scene. Also, there are three water balloons left after they hit Mitch in the face, but when the scene cuts back to Roger after Mitch's car explodes, there's suddenly only one.
The red flower Stan pins to Betty Sue vanishes.
Roger's military jacket has a red badge on the right shoulder and nothing on the left, but when he and Mitch finally find Steve on the porch, the red patch has moved to the left shoulder.