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.
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. Its hard to believe no one in a planet of outlaws would try and attack them for the orb to get the money themselves.
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.
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.
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. We don't see her leave.
On Nova Core when Gamora is chasing Quill she jumps from the upper level to a patio below. She clearly lands with both feet close to each other. Cut to a shot zooming away from her and her feet could hardly be further apart as she is nearly doing the splits sideways.
When Quill and Gamora are introduced to The Collector, there are two men flanking either side of him. As Gamora approaches The Collector, you no longer see the two men in that shot or in any subsequent shots.
As Star Lord is entering the temple to steal the orb he grabs and picks up one of the rat creatures. In the close up shot that follows he uses the rat as a microphone. In the next long shot he has no rat, next close up shot the rat is back. This repeats again.
When Peter Quill arrives to the Kyln and is stripped to his shorts and washed with the orange stuff we can see he has no scar and has a six pack. Later that day he goes to the showers to save Gamora, he lifts his shirt to show some scars to Drax and there is no six pack. (Incorrectly regarded as a goof was that he had no abdominal hair. It is momentarily seen before he is sprayed with the orange stuff. The hair blends in with the color and lighting after the wash.)
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 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'.
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, you can see that 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.
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.
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.
After Gamora tells Quill his ship is filthy, he says to Rocket that if he "had a blacklight, this place would look like a Jackson Pollock painting". While Quill was approximately 10 when he was abducted from earth, this is not too young to learn about artists. Many children who have few friends and are bullied at school, as Peter stated he was, learn about artists, especially experimental artists such as Pollock. Blacklights are universal and he could have learned about their reactivity to bodily fluids amongst his travels with the Ravagers.
The weapon used to threaten Yondu and against Ronan is called a "Hadron Enforcer." The name is supposedly derived from the Large Hadron Collider, a scientific tool that smashes particles together almost at lightspeed. The name is somewhat incongruous, as the weapon does not use near-light-speed particles for destruction, instead using an extremely explosive bullet/missile. However, it should be noted that the weapon was named by Rocket Raccoon himself, mere hours after he finished building it ("That's for if things get really hardcore..."). The technical inaccuracy of the name is more indicative of Rocket's limited knowledge than anything else.
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.
The goof items below may give away important plot points.
While they are trying to hold off Ronan's ship, Rocket tells the Nova Corp that the city has been evacuated, but after Ronan's ship crashes to the surface there's a substantial crowd forming around them.
When Quill dives through the air to catch the Infinity Stone, he's reaching for it with his right hand, but when he stands after he's caught it, it's clutched in his left. It's hard to see, but when he's writhing on the ground, he tries to hold the stone with both hands, can't do it, and switches it to his other hand.
When Gamora is floating in space, Star Lord flies out to give her the helmet. Shortly after, Yondu brings them onto the ship, Gamora and Star Lord don't have the helmet. However Star Lord's helmet retracts to a small device behind his ear. After he puts it on Gamora, it just retracts back to its smaller form.