Continuity mistake: In the beginning of the movie when the guardians are getting ready to fight off an interdimensional 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. (00:03:45)
Revealing mistake: When Taserface is threatening Rocket, Rocket is making eye contact. Taserface, meanwhile, is looking at the space between Rocket's ears.
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.
Continuity mistake: While Yondu is explaining how he tracked the Milano, and who sent him and why, look at the 3 arrows pinned to Yondu's jacket when he first appears. Once he starts his walk around while explaining, there are only 2 arrows. After the scene returns from Nebula and baby Groot, the shot seems to be mirrored, with the three arrows in a different position. (00:40:15)