The position of the scar on Rambo's face has changed. In Rambo: First Blood Part II (and Rambo III) the scar was more vertical, and started around the corner of his eye. In this movie it is more diagonal, and starts around his hairline. This is ironic, since when Rambo is dreaming, amidst the montage of flashbacks, he actually flashes back to the scene in Rambo: First Blood Part II when he initially received the scar, and you can clearly see its position on his face.
After the final gun battle, Sarah leaves her position with the sniper, and we see her move quite a distance away as the sniper lifts his rifle. As they cut to a different angle of the sniper relaxing/packing up, we can clearly see Sarah's head/hair in the lower right corner of the screen, only a foot or two away from the sniper.
The machine gun mounted in the front of the Burma Army's Patrol Boat. The first view of the machine gun, there is a double-drum shape magazine mounted on top of the gun; minutes later, the second view of the gun, there's no magazine to the right side of the machine gun; seconds later, the third view of the gun, an ammo box as well as a box-holder shown up to the right side of the machine gun; moments later, the fourth and last view, the ammo box along with its holder go to the left side.
The Claymore mine changes from a training Claymore (Blue in color) to an Airsoft Claymore (OD Green but with a pellet loading hole in the front cover). The tripwire firing mechanism also changes between shots, but was not present when Schoolboy tossed it to Rambo.
During the scene when Rambo is going to lead the tracking dogs away from the girl and Schoolboy, Schoolboy's hat brim keeps changing how its bent in between shots. It changes from peaked to flat to a round shape.
Sarah is handing supplies out to the villagers. She comes up to a little girl, with a package in her right hand. It goes to another camera angle, and the package is in her left hand. It cuts to a different camera angle, and the package is back in her right hand.
After Rambo fires the last arrow from his compound bow, he regroups with the mercenaries. As he approaches them the bow-string is seen drawn back without being held by the release hand. This could only be done if an arrow were placed on the string, but there is no arrow mounted to fire, even though seconds later he holds a drawn arrow inches from the mercenary's eye.
In the final gun battle sequence, Rambo uses the jeep mounted gun to fire on the driver, and splatters blood all over the bullet shield. When he gets off of the machine gun, the bullet shield is clean.
The M-60 gunner on the bow of the patrol boat gets his head blown off (literally), yet his body is still at the gun, hand gripping the trigger. However, a moment later when Myint hits the boat with the rocket, the M-60 gunner's body is nowhere in sight.
Towards the end of the film as Rambo makes his way through the police station, there is an air vent swaying back and forth after having being shot. As the camera pans,a crew member is visible manipulating the object to create the effect.
When Rambo kills the gunner on the jeep-mounted machine gun, and takes over the weapon, the first thing he does is lower the barrel and blow away the
soldiers in the forward seat. This would not be possible. In order to prevent
this from happening accidentally in the heat of combat, the gimbal on which the gun is mounted would have a built-in mechanical stop, which would prevent the gunner from lowering the barrel far enough to kill his own crew. This safeguard was applied on WWII bombers, to keep the side and turret gunners from shooting into the wings and engines of their own planes.
The large bomb found in the jungle is not a Tallboy as previously suggested, but a "Grand Slam", the successor to the Tallboy bomb. The key indicator is the overall size of the larger 22,000 pound bomb, and the aerofoil shape of the stabilizing fins, versus the relatively flat fins of the tallboy.
It's perfectly plausible that such a bomb could sympathetically detonate after 60 years in the jungle. The explosive filler is a cast RDX/TNT based explosive (torpex) which is more or less impervious to the elements. While it's entirely possible, and quite likely that the secondary booster explosives would have deteriorated, and indeed the very fact that the bomb did not explode when dropped would suggest that the fuses were damaged or misused, it's entirely possible that a very large container of torpex could be sympathetically detonated with a C4-based claymore. The implausible aspect of the bomb is that, indeed, they were only used in Germany, were extremely rare and expensive, and could only be dropped from a fairly specialized Lancaster bomber.
When Rambo is using the .50 caliber machine gun in the final action scene, it runs out of bullets and makes several clicking sounds to indicate it is empty. The gun would only click empty once since the gun's action only operates by firing, thus cycling the round and cocking the action.
In the final shoot out Myint fires a M136 AT-4 anti-tank weapon although the weapon is still on safe and he has not got his finger on the trigger. He also holds the weapon totally wrong and looks over the sights.
When the team attacks the Burmese camp and the sniper fires the first shots, we see a guard falling backwards with his legs rolling onto him. Later when Rambo is getting out the camp and is spotted by 2 guards, as the second guard is sniped, we see someone falling exactly in the same manner with legs rolling onto the person - it is in fact the same shot that we saw earlier, only with a wider view.
The goof items below may give away important plot points.
At the end of the movie Rambo kills the last bad guy by stabbing him in the stomach with his knife and slicing him open, yet a moment later when Rambo kicks him down the hill, there isn't one drop of blood on his knife.
During the ending gun battle scene when the camera switches to view the right side of En-joo (Tim Kang) you see a shot of the camera equipment behind him. (This is when he climbs up the steps to the upper tower for cover and better shooting position).