D.B. Cooper's partner could not have known him as DB before the flight. The hijacker called himself Dan Cooper, which was only changed to DB after his presumed death, from a reporter's bad handwriting.
When Dan, Jerry, and Tom are driving 'the duke' to go on their expedition, Dan refers to 'the Duke' as having "the 318 V8, they're bulletproof." Wagoneers were never offered with a 318in3, especially not in 1977. The 318 was a Chrysler motor, and in 1977 Jeep was still under the ownership of AMC, and would come with either a 258in3I6, a 360in3V8 or a 401in3V8.
When the 3 guys are in the tree house, they say the toys won't be taken out of the box until they find the treasure. When they have just gotten off the rapids, Dan has his little metal man, and then Jerry has the Indi-Compass.
After the funeral, Tom, Jerry and Dan go back to their old tree house. They find their old treasure box and the camera shows a shot of the box with the lock in the hole but clearly unlocked. Then the say that they can not get into the box but Dan tells them the combination and then they say that the combination worked even though it had clearly been unlocked before he tried the combination.
After the pot farmers are bombed with the poo bags from the treetop and have taken off after the boys on the all-terrain vehicle, the crap they have been bombed with is suddenly missing from their clothing and faces.
When the pot farmers confront the guys at the tree house Tom sets up a zip-line out the back for Jerry and Dan to escape on. When Jerry's preparing to go down he has a piece of cloth to hang onto but when they show him sliding down the line you can see a trolley hooked up to the zip-line.
In the scene just after the funeral where they enter their old tree house it is dark outside but after Dan enters the tree house you can clearly see that it is daytime outside through the cracks and gaps in the wood.
When the guys are in the tree with the girls, they call for help on a CB radio. The bad guy hears and responds to them on an FRS radio. This is impossible since the CB works in the 27 MHz range, and the FRS radio works in the 440 MHz range.
When Dan's receptionist enters the room to tell him that he has a phone call from Jerry, the film is reversed. You can tell this because of her name tag and because the phone he picks up in the next shot is the same one that appears next the her on the wall.
After the guys regain safety from the rapids, and unflip the canoe, they appear to be in calm water. Seconds later they are back in rapids again, and fall down a waterfall, there would be no way for the water to be calm right in front of a waterfall.
When Del is shooting at Dennis and Elwood, sparks from the ricochets are seen on the fenders. This isn't possible because the fenders, like most of the ATV's panels, are made of plastic. Also, even if they weren't, bullets are made of lead, which won't cause sparks even against metal objects.
The goof item below may give away important plot points.
Near the end when Elwood, Dennis, and Sheriff Briggs are about to kill Tom, Jerry, and Dan, Elwood's gun suddenly changes from a rifle to a handgun in a closer shot, and then back to a rifle in a wider shot.