Continuity mistake: In the beginning of the movie when the guardians are getting ready to fight off an inter-dimensional being Rocket winks a couple of times at Starlord. Starlord then responds but is looking to his left at Rocket, whilst Rocket is on his right side. The scene is for no reason mirrored, proven by the big orange device behind him also switching from being on his left to on his right.
Factual error: At the very beginning of the film it says that it's 1980 Missouri with a nuclear power plant in the background. The plant in the film has 4 cooling towers, but the only nuclear plant in Missouri (Callaway Nuclear Generating Station) has only one cooling tower and it didn't begin operation until 1984.
Ego: After all these years I found you. My name is Ego, and I'm your dad, Peter. I wanted to experience what it was to be human. So I created what I thought a biological being would be like.
Drax: Did you make a penis?
Peter Quill: Shut up!
Ego: Yes, Drax, I got a penis. It's not half bad, either.