The alien attacks Bill and Brian at the ranger's station. Bill escapes in his truck. The alien, walking very slowly, beats him back to the house.
The movie frequently jumps back and forth between day and night, sometimes in the same shot. It's most noticeable when Joe wonders aloud who could be banging on the front door in the middle of the night. In the previous shot, when Rick was standing outside, it was clearly daytime.
When Rick finishes singing in the recording studio, he takes off his headphones. Just before he gives the thumbs-up to the backup singers, he takes off his headphones again.
As Bill and Brian are driving to find help, the amount of snow on the ground varies wildly between shots; notably, almost none is visible during the character closeups.
When Robbie is showing Trumpy how to play Simon Says, the tones played for the button prompts do not match up with the on-screen lights.
Despite being born on Earth less than a day ago and having had no contact with any of his own specie or humans with the exception of the boy Tommy, he somehow understands english and is aware that he's an alien.