(at around 10 mins) When Peter is escaping Korath in his ship on Morag and the ship is upside down, he smacks into the windshield. In this shot, he has no gloves on, where moments before he did. In the next shot, when he's reaching to grab the ship's controls, he has his gloves on again, and they remain on for the remainder of the scene.
(at around 1h 50 mins) The text of the note from Peter Quill's mother does not match the voice-over.
In one scene, Nova Prime's hair style changes between shots.
(at around 26 mins) At the Kyln when the Guardians are dealing with the Monstrous Inmate, Gamora is behind Quill when she speaks. When it cuts to a wide shot and all the shots that follow, Gamora has disappeared.
(at around 17 mins) While Quill is dealing with the Broker for the orb, the Broker is lining up yellow and blue Units on the counter, three yellow ones followed by two blue ones. After placing the second blue one his hands move to his side just as Quill mentions Ronan. When the camera switches behind the desk again, despite not having touched them, one of the blue Units has disappeared and the order has changed from a single line to a double line, with the yellow Units on the bottom and the remaining blue Unit on the top.
The "Guardians" are arrested on Xandar and taken to the Kyln. When they make their escape, they do so in Quill's ship. Quill's ship would not have been transported to the prison from Xandar. Also, Quill was not with his ship at the time of his arrest so authorities wouldn't know about his ship at all.
(at around 22 mins) When Quill is captured by the Nova Corp and he is in front of the window being reviewed, on the text on the window to the left they list his alias as "Space Lord" not "Star Lord". This is part of a running gag in the film where no-one can remember Quill's 'codename'.
(at around 34 mins) When Rocket is explaining his prison escape plan and Groot goes to get the battery mounted on the wall, Groot's food tray disappears from his right hand as it passes behind Peter's head. However, his body leans to the right as he passes behind Peter in the frame and a clinking noise can be heard, hinting that he dropped the food tray off onto the table.
(at around 1h 6 mins) In a perfect a vacuum, no sound can be heard; but when Peter is rescuing Gamora the sound of his boots is heard, as well as other things. This is due to the fact that Peter and Gamora are still well within the gravity well of Knowhere and so, still surrounded by the faintest of atmospheres.
(at around 59 mins) While visiting the Collector, he places the orb on a device that opens the orb. When Carina grabs the Infinity stone floating inside the orb she is clearly holding the stone before she instantly explodes and does major damage to the Collector's room and collection. Because she is not strong enough to hold the stone, her hand never moves the stone. As she explodes, several camera angles of the scene show the stone remains in the orb's force field the whole time. So the stone never needs to be placed back into the orb.
(at around 1h 15 mins) The weapon used to threaten Yondu and against Ronan is called a "Hadron Enforcer." Despite the thought that the weapon should have a near-light-speed component, due to bearing a similar name to the Large Hadron Collider, a Hadron is actually just a subatomic particle made of two or more quarks bound by the strong nuclear force and has no speed requirement. The particle name was coined in the 1960s (well before the LHC) and include the universally common protons and neutrons. Thus, there wouldn't necessarily be any incongruence or inaccuracy in the name, since the technology behind the weapon wasn't explained.
(at around 1h 40 mins) After Ronan's ship crashes and Quill/Star Lord gets the Infinity Stone, Gamora takes one of the "spare" orbs and slaps it down on the Stone in Quill's hand. She does not open the orb, she just slaps it on top of the Stone and the Stone jumps inside the still-closed orb.
(at around 7 mins) At the beginning, when Quill is going through Morag, searching for the orb, he is walking across dirt and sand - in one shot, the 'floor' he is walking across is obviously square, a piece of plywood that's part of the sound stage.
When Gamora releases the security lock on the door, she then shoots up in to the ceiling to enter the same room where Ronan is located so she could join the rest of the Guardians, making the need to unlock the security door pointless.
(1 hr 51 min) when Peter adjusts the volume on his tape deck, the VU meter moves with it. This meter is only adjustable when recording a cassette, not playing one.
The 'awesome mix' cassette tape shows part of a distinctive logo under the label from a TDK cassette tape type II , 'CDing 2' . This particular brand of cassette tape first came out in 1993, yet the start of the film is 1988.
After Ronan's ship crashes and Quill/Star Load gets the Infinity Stone, Gamora takes one of the "spare" orbs and slaps it down on the Stone in Quill's hand. She does not open the orb, she just slaps it on top of the Stone and the Stone magically jumps inside the still closed orb.
During the final battle, Gamora explains that Ronan will secure himself in an impenetrable safe room. She must destroy its power source to allow the other Guardians access. However, once she does this, she simply shoots a hole through the floor with a rifle and joins them in the "impenetrable" safe room.
If Ronan used the Infinity Stone to destroy Xandar and everyone on it while he himself is on the planet, he would destroy himself as well.
The Collector had received the red Infinity Stone at the end of Thor: The Dark World. Given he knew how powerful and valuable those artifacts were. Is really odd that he didn't have a plan to use the red stone to protect himself or given the amount of money he had, at least hire a proper team of body guards or private security. Specially since he was in a place full of outlaws. He should had expected someone would attempt to steal the red stone from him, at any moment.
(at around 54 mins) When entering Knowhere it is said to be a planet of outlaws with little to no order. Later during the big fight of the Guardians, Peter Quill gets the group to calm down by yelling they are about to make a trade for 4 billion units. It's hard to believe no one in a planet of outlaws would try and attack them for the orb to get the money themselves.