At the military camp where the prisoners are held, it rains all night. The next morning, when everyone leaves the camp, the ground is dry - even dusty.
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.
The position of the scar on Rambo's face has changed. In Rambo II - Der Auftrag (1985) (and Rambo III (1988)) 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 II - Der Auftrag (1985) when he initially received the scar, and you can clearly see its position on his face.
While on Rambo's boat, En Joo has a full beard with sideburns and a thick mustache. When they land, his sideburns are shaved and his beard and mustache trimmed.
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 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 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.
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.
In the rainy camp rescue scene, schoolboy's sniper shots are "silenced" although his sniper rifle does not have a suppressor.
Incorrectly regarded as goof: While the Barrett .50 cal does not have a suppressor in the daylight chase scenes, and only sports a muzzle brake, you can clearly see one is attached during the nighttime scenes. It is especially visible when he is climbing into the tower and getting into position. He most likely removed it afterwards because the extra length would make running quickly through dense jungle foliage more difficult. A suppressor on a Barrett can effectively reduce muzzle noise/flash at the cost of increased recoil, however the high velocity round still creates a loud sonic boom.
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.
During the arrow shooting scene, when Rambo shoots the soldier reaching in the truck, a soldier can be seen leisurely standing around in the background like nothing is going on, even though it had been several seconds into the ambush.
Rambo's boat has a hidden inboard motor at the stern, you can see the water churning from under the boat in views of the rear of the boat.
In the final gun battle sequence when Rambo goes to reload ammo for the jeep mounted .50 cal machine gun, he grabs the full ammo box and it clearly says 840 Crtg 5.56 ball M855 10 rounds Clips. This is not the correct ammo type, style or ammo box for the weapon he is using. Though the Burma regulars may be using surplus ammo boxes it would make a logistic problem trying to keep track of what ammo you have if everything is mismatched. Seemed odd. Thanks.
When John Rambo is passing by the empty boat that had previously attacked his party, the motor is clearly not running in one shot since the front pulley is not turning (23:54).
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.
In the beginning map scene, CALCUTTA is clearly spelled CALCUTA, indicating that the map was prepared in a foreign language such as Spanish.