Bowman empties his gun and the slide stays cocked back because it's empty. In the next scene the slide is back in position, but Bowman wouldn't know how to do this to a gun, having not used one before.
The Polaroid which terry examines with a magnifying glass shows Peter smiling slightly. The picture Alice & Janice look at has his mouth closed in a grimace (the plot makes an issue of why he isn't smiling in the photo).
In the final scene, when the helicopter is returning from the retrieval mission, Meg walks up to greet her husband, and she has a huge fever blister on the right side of her upper lip. Later, in that same scene, the fever blister disappears.
When Peter is photographed by his captors, his feet are completely red, covered in blood. Then when the instant-photograph is being looked at in the hotel room by Terry, Alice and Janis, there is less blood and injury visible on his feet.
When the rebels arrive at the camp with Peter in their midst, you see the guy with the yellow shirt, Alex, as his fellows help him to lie down on a table. A short while later Alex arrives at the table, helped by his mates. And later still we see him dismount the mule.
When Kessler is standing at the top of the waterfall (shown in a long shot), he clutches at his injured shoulder. However he has not been shot yet. He gets hit in the shoulder during the next (close-up) camera shot.
The grenade Dino throws into the hut is a noise/flash distraction device, commonly called a "flashbang." Flashbang grenades do not detonate and fragment like antipersonnel grenades, so it would be normal to see the spent grenade body lying on the floor.