In the first chase where the police of the Anubis Universe are chasing Yulaw, the first car goes under the partly raised garage door and hits its lights on it. The siren dies and the light and siren get broken off. When the car drives on, however, the top is sparking but the lights and siren are intact.
While Gabe is practicing Bagua Zhang in his den, he actually slides over the wooden paneled floor with his Magnum Boots. It's impossible to slide with these things (especially on wood) since the carbon rubber sole is skid-proof.
The goof items below may give away important plot points.
At the end, when Gabe Law is in his new universe, he sees a dog, that has been hit by a car, lying on its right side on the street. When he picks up the dog, its right front paw is bent back, hinting that it is injured. When Gabe brings the dog into the vet clinic, its right front leg is straight and Gabe seems to place the standing dog on the counter top.
Toward the end, during the final fight scene, when Gabe is dragging Yulaw to the wormhole and holding the ax to his throat, you can tell the head of the ax is some sort of foam, as you can see it bending against Yulaw's neck