When Charlie is driving to the Road to Nowhere, at first he's wearing a white tank top, then for the rest of the driving scene he's wearing an olive green t-shirt. When he arrives at the entrance and jumps out of the car, he's wearing the white tank top again.
When they fall off the cliff near the end, the stunt mat is clearly visible at the bottom of the screen.
When Charlie enters the road to nowhere and observes the gaps in the rocks above. The camera tilts up and a light appears in shot to the left of the screen.
When Charlie jumps the police car across the chasm and into the cave you can clearly see the tops of blue sandbags lined across the bottom of the screen when the car stops.
When Ben Stiller makes the "Hell Cop" motion resulting in the coffee pot lid blowing off, you can see the black rubber hose used to make the stunt work running along his arm.