Continuity mistake: When Karl Urban is defending the Asgardians, the dust covers on his rifles vary between being open and closed several times. M16 dust covers are sprung loaded - they open on the first shot and have to be manually closed afterwards.
Continuity mistake: After Peter gets the text from Happy about meeting at the bathroom, Michelle asks, "What are you hiding, Peter?" and just as she says, "I'm just kidding I don't care," Peter's holding his cell phone up in his left hand, but next shot he's holding the cell phone up in his right hand.
Continuity mistake: During the opening scene with Ego and Quill's mother, they run into the woods behind the Dairy Queen, and mom is wearing boots with fur trim. When they get to where the Ego's plant is in the ground, she is suddenly wearing sandals.