Revealing mistake: When Cap is driving the blue Chevrolet Silverado with Black Widow, the gear shift is in the up locked position, not lower on the column, indicating the truck was towed in the scene. (00:54:45)
Revealing mistake: When Tony goes to the bathroom and scans his blood toxicity, the Tony who faces the camera is supposed to be his reflection in the mirror, but the problem is the reflection is not reversed because it's not a mirror image, note Tony's clothing. Also, note how he scanned his right hand's finger, but then in the "mirror" he's sucking on his left hand's pricked finger. (00:28:30)
Revealing mistake: When the Hulk walks through the stream, the water doesn't react to his feet. (00:59:20)
Revealing mistake: When we see the new recruits at boot camp, they are wearing M1 Helmets. But if you look closely, they have rivets through the sides of each helmet where the chinstrap bales would be. In reality, the M1 helmet chinstrap bales were and continued to be welded onto the shell of the helmet. This indicates the helmets in the film are probably plastic replicas with the bales being riveted in place.
Revealing mistake: Some of the shots involving the terrorists firing at the prototype suit are "flipped" duplicates of other shots. In the scene where he first emerges from the cave, several of the terrorists are firing M4-type rifles with the ejection port on the LEFT side of the rifle. In reality, except for a few specially made versions, all M4-types eject spent brass through an ejection port on the right. Later in the scene, this shot is repeated, but with the proper orientation.
Revealing mistake: You can see the blue screens on Bruce Banner's glasses when Tony Stark is talking to him about embracing the Hulk rather than disliking it. (00:57:40)
Revealing mistake: When Captain America is fighting Ultron in the truck, for a brief moment you can see a safety line holding actor Chris Evans as he's hanging on the side of the cab of the truck.
Revealing mistake: During the fight in the forest with Heimdall and Hela's soldiers, he uses the sword but the tree is cut before the hit, and the soldiers on the right fall before the hit too.
Revealing mistake: During the face-off between Bucky, Captain America and Iron Man at the end of the film the fake snow is seen stuck to the actors after they fall down. It remains throughout the scene rather than melting as real snow would.
Revealing mistake: Just after Proxima Midnight and Corvus Glaive land in the train station, there is a wide shot where they begin approaching Wanda and Vision. In the wide shot, their shadows do not appear to match up with each other, despite being under the same lighting. Proxima Midnight's shadow is positioned in front of her and is quite heavy, but Corvus Glaive's shadow is positioned more to his left and is much lighter than Proxima's. (00:40:20)
Revealing mistake: After the Kree arrive on the ship, in a side shot one of them opens the lunchbox holding the tesseract. We see the blue light emanating from inside, but that only appears once the box is fully open - while it's opening, there's no light inside at all, despite the tesseract always glowing. A later shot shows the glow being visible around the edges of the lid even when the box is completely shut, and when Carol opens it later, the glow appears immediately.
Revealing mistake: When Tony and Rhodes are on the oil tanker bunkered behind an obstacle, Tony realises his gun is empty and asks Rhodes for a new magazine. When Tony ejects the magazine you can see that it is actually an airsoft gun magazine; thus exposing that it is actually a prop gun.
Revealing mistake: When Taserface is threatening Rocket, Rocket is making eye contact. Taserface, meanwhile, is looking at the space between Rocket's ears.
Revealing mistake: When Scott is fighting Ghost, as Pym is planning to dive, Ghost throws Scott through a wall and you can see the braking cable around his/the stuntman's waist and across the floor. (01:16:48)
Revealing mistake: During the attack on Druig's village, there's a shot of Gilgamesh and Thena running towards the camera, then stopping as two Deviants fly over them. During this scene, you can see Don Lee (Gilgamesh) briefly look straight at the camera (he's not looking at either the village or the Deviants, since they headed in the opposite direction). (01:13:50)