Continuity mistake: It's shown in many scenes that the robot Gallaxar sends isn't particularly quick moving when it walks. But when it's chasing Ginormica around the city, even though Ginormica is sprinting away, it can catch her far more quickly than its speed would allow. It doesn't suddenly speed up - in the shots when the camera focuses on the robot again, you can still see it moving slowly.
Plot hole: When the asteroid first approaches Earth it does so very quickly, but then it takes hours to finally enter the atmosphere and land on Susan.
Factual error: The TV station in Modesto, CA had call letters starting with a W. All broadcaster call letters west of the Mississippi start with a K.
Revealing mistake: Susan appears to pass right through the glass window after knocking down Gallaxhar.
Continuity mistake: When the President is climbing the stairs to make first contact with the robot, the man who gives him the cup of water is nowhere to be seen. This is especially evident from the view of the helicopter pilot who says "we got a bead, Papa Bear." (01:00:00)
Continuity mistake: The man running up to the door for the joke about having to have so many things scanned, shows separate scanners being used for his hand, both elbows, tongue, butt, etc. When he's running up to the door, it shows there is only one scanner. But there are multiple in the same spot as the door opens for him.
Other mistake: When Derek does his final news report the map in the background says both 32 and 99° in Nevada.