Factual error: Throughout the whole movie, whenever federal agents (or people posing as such...) tasked with the case are shown, they wear uniforms with "U.S.A. Marshals" written in big yellow letters. Of course the one and only correct spelling would be "U.S. Marshal"
Continuity mistake: When Pete is holding his hands out, as per Ego's instructions, the orientation of his hands changes. Initially, his thumbs are roughly at the top, but when the camera angle changes, they are at the side. No time has passed, and Ego didn't tell him to change anything.
Continuity mistake: In the mid-credits scene in Justice League vs. Teen Titans, Terra's hair was shoulder length and she had no freckles, but in this movie, she now has shorter hair and freckles.