Scene 1: Doctor Strange and Levitation
One of the simpler defiances lies in the power of levitation or unaided flight. Many characters like Iron Man or Star Lord have rocket propelled forms of flight, meaning there is a force keeping aloft, however, for characters like Doctor Strange, they have no force keeping them in the air. According to Newton's Second Law, gravity acting on a living being with mass (which Doctor Strange definitely has) while in the air means he should be falling downwards. Since Doctor Strange does not
have a downward facing force to keep him in the air, he should be falling to the ground but since that is not portrayed in the movie, this counts as a violation.
Scene 2: When He Should Have Gone For The Head.....
In this scene, Thanos has just killed Vision and inserted the Mind Stone into the Infinity Gauntlet. In an attempt to prevent the Snap, Thor flings Stormbreaker into Thanos' chest but fails to kill him. It instead gravely wounded him, but not enough to prevent Thanos from killing off half of all life in the universe.
The problem with this scence is that when Thor hit Thanos with Stormbreaker, it should have created a crater or indent in the earth of some sort but it didn't. This violates Newton's Third Law of Motion. The downward force created by Stormbreaker is enough to create a crater as evidenced by a previous scene where Thor slams down is hammer and creates one. The same thing should have happened with Thanos. The force from the impact would cause an opposite upward reaction from the ground around the impact point which leads to the formation of the crater. Even though in this scene Stormbreaker hit Thanos and not the ground, the force would have channeled through him and still have the same effect.
Scene 3: The Hulk in the Hulk Buster
(Sorry this one is a link! It was the only way to get this scene in!)
In this scene, Bruce Banner has been equipped with the Hulk Buster armor, a huge metal suit designed to defeat the Hulk in violent confrontation. He is running across a grassy field towards the front lines of the battle that is about to commence. But again, the physics in this scene is screwed. Much like the previous scene, this one is a violation of Newton's Third Law. It is violated by the non-existence of an impact craters. The Hulk Buster armor is a giant heavy metal contraption that is exerting a good amount of force on the ground beneath him as he his running, but no crater (aka the opposite reaction) is created which in fact should be present.