Continuity mistake: When Buddy's owner is chasing Josh and Buddy in his truck, there is a scene that shows the truck approaching the water which is a couple of hundred feet away. As the camera cuts back to the driver, he does way too many things before the truck goes in the water (slams on brakes, steering wheels falls off). When about 10 seconds has lapsed, the exact same camera angle is shown of the truck being a couple of hundred feet from the water. The truck would have already been in the water by then.
Continuity mistake: In the scene where Josh is making a doghouse for Buddy out of a cardboard box, he puts a smiley face on it. In the close-up, he draws two long lines for eyes, and a thin smooth curve for a smile. A second later, the eyes are dots and the smile is drawn in a heavy, messy line that's thicker on the left side than the right.
Other mistake: In the part where Josh is sweeping the basketball court behind the church you notice that there are faded white paw prints. Then later when Josh is painting it Buddy walks directly on the white prints. Were these there to show Buddy where to walk?
Continuity mistake: Throughout the whole movie the weather changes. They would show it being sunny when the kid is running to the fence then when they switch camera angles and he's on the other side of the fence it is cloudy.
Continuity mistake: At the beginning of the film, we saw the box with the dog falling down in the street. The road is dry. When the truck comes the road is wet.