Revealing mistake: When the Mack truck is trying to run Taggart off the road, the scenery on the copilot's window is an obvious chroma: first, it doesn't match the wide shots (bright green background vs cloudy and dull); second, it is repeated a couple of times. Also, when Taggart's car is cornered next to a mountain, the close-up shows a blue background.
Revealing mistake: During the opening scene, a few shots after we see the Outworld ninjas falling through the portal and leaping about, there's a shot of several monks fleeing that tilts up towards the (digitally replaced) sky. As it does this, you can see several hard lines about in the sky from where pieces of the digital animated skies were "stitched together." It's easy to miss the first time, but if you know to look for them, they're pretty obvious. You can see similar lines in the sky in several other shots.