When Thor is explaining to the Guardians of the Galaxy where they need to go to forge a weapon to kill Thanos, he pronounces the destination as "Nivadellir" instead of "Nidavellir". He says it wrong twice in that scene. Yet, In the third act of the film he correctly pronounces it as "Nidavellir".
While on the Maw's spaceship, Tony Stark, Peter Parker and Dr. Strange's cape have a conversation. and for the entire scene Peter's hair is slicked back. During an intense exchange with Tony, Peter says "You can't be a friendly neighborhood Spider-Man if there's no neighborhood"; the camera cuts to a speechless Tony and then goes back to Peter, who now has a completely different hairstyle, featuring a fringe. His hair turns back up for the rest of the scene.
The first time we see Tony Stark, he is walking with Pepper Potts in a park, discussing. Tony has a hoodie on his shoulders, its sleeves tied in a knot covering the miniature arc reactor on his chest. During their exchange, Pepper unties the knot to talk about the device on Tony's chest. Right after that, cutting from a three-quarters shot to a close up on Tony, the hoodie disappears and is nowhere to be seen for the remainder of the scene, up till the moment we are taken inside the Sanctum Sanctorum.
In Guardians of the Galaxy Gamora is described as the last living member of her species, in this film Thanos is shown to have killed only half of her planet's population. Indeed he tells her that life has been better for those born on her homeworld since his genocide.
When Peter falls from the spaceship in the upper atmosphere, the rocket containing the Iron Spider suit makes contact with his back and the suit begins spreading outward. The spider on the back of the suit is visible in one shot, then in the next shot, it shows the back forming (even though the back was the first part to form).
During the New York confrontation, when Ebony Maw strangles Dr. Strange and he falls down, he hits a cop car next to him. In the next scene, when Ebony Maw lifts the ground around Strange, the cop car is nowhere to be seen.
Tony Stark's mustache changes as he is confronting Thanos on Titan. He says hes "gonna lose it" if Thanos throws another moon at him. In this shot, his mustache is dark and defined. After Thanos calls him by his name, in the very next shot his mustache has a lot more gray in it and is a bit thicker/unkempt.
In the post credit scene we can see Nick Fury and Maria Hill acting as they are still agents of S.H.I.E.L.D. Because of the events of Captain America: The Winter Soldier and the Agents of S.H.I.E.L.D. TV series by this time S.H.I.E.L.D. is a wanted criminal organization with very few active members.
When Ebony Maw wraps the electrical cable around Strange, he loops it several times around his neck to make him pass out. Immediately after, the cable is no longer wrapped around his neck, allowing his cape to fly him away. The cable is also tightly wrapped around his body, and then loosely wrapped around it in the next scene.
When Stark, Strange and Banner first encounter Ebony Maw and Cull Obsidian on the street, there is one yellow cab and one hidden car between them. When Cull Obsidian walks and then charges, several cars are suddenly between them, giving Banner and Stark lots of time to react.
According to the end credits, one of the production units filmed both in the United Kingdom, and in Scotland. Since the United Kingdom includes Scotland, that would be like having a unit that filmed both in the United States, and in Missouri.
Allowing suspension of disbelief that Tony Stark's chest piece could hold enough nano-material to form the suit and all of the missiles launched from it, it is absurd that it would contain enough material to plug a six-foot hole in a spaceship hull.
When Thor goes to Nidavellir, Eitri tells Thor he made the gauntlet for Thanos because Asgard neglected to protect them. Asgard was just destroyed prior to the first scene of this movie, and yet the gauntlet was shown at the end of Age of Ultron, which takes place years prior.
However, recall that Loki had been ruling Asgard for four years (overlapping Age of Ultron), and had allowed the Nine Realms to fall into war and chaos without doing anything on his own part to protect them (as opposed to Odin always striving to protect his kingdom). Loki's neglect would have most certainly left Nidavellir exposed to an attack from Thanos (assuming that Thanos obtaining the gauntlet in Age of Ultron is chronologically correct), which may be what Eitri was referring to.
In Black Panther, when King T'Chaka shows up at the beginning, N'Jobu and James (Zuri) bow before him to revere him as a king. In this movie, however, Bruce Banner attempts to bow before King T'Challa, who then states "We don't do that here." However, in the same movie (Black Panther) when Shuri bows for T'Challa in modern-day Wakanda, he recognizes this as a joke and tells her to stop. This could suggest that he had changed reverence policies since his father's reign.
When Tony Stark first uses the nano tech to put on the Mark L armor he takes off his shades with his right hand. As the armor finishes you can see the shades disappear from his right hand. Fully formed his right hand is still holding the now non existent shades. However, this is not a mistake. If you pay close attention to the glasses you will see that the shades are actually nanobots that are absorbed into his suit.
The goof items below may give away important plot points.
After The Snap, the first person to turn to dust is Bucky. He does so while he is walking towards Captain America with a gun in his hand. When he turns to dust, the gun which is an inanimate object falls to the ground. But his machine arm, which is also an inanimate object, turns to dust.
When Rocket gives Thor the eyeball and the two are discussing it, Rocket is standing next to Thor in one shot but appears in the pilot's seat, fully seat-belted in the next shot, even though audio seems to indicate the conversation between the two is continuous.
When Thanos turns back time after Scarlet Witch has destroyed the mind stone and Vision is "revived", Scarlet Witch tries to stop him while shouting. In the next shot you see Thanos hitting Scarlet Witch with his right hand and you see him holding vision by his neck with his left. In the next shot you see Thanos reaching for vision and grabbing his neck.
In the opening scenes on the Asgard ship Thanos uses Corvus Glaive's staff to kill Heimdall by stabbing him through the heart and it can be clearly seen in shot with Hiemdall's face as. However when Corvus removes his weapon he pulls it from Hiemdalls stomach.
After Groot makes a handle for Stormbreaker, it is shown falling to the ground with the sharp side on the right and the hammer side on the left. When it lifts off the ground to go to Thor, the head is reversed.
After Thanos is victorious, people throughout the universe disintegrate with their remains blowing away. This includes their clothing and anything else they are wearing. However, other inanimate objects survive, such as Fury's communication device after he sends a distress signal to Captain Marvel. However, Nick Fury is shown to throw the distress signal on the ground before his hand disintegrates.