Revealing mistake: Bruce Willis wore fake feet during the movie to protect his own while shooting the film. They were like boots that slipped on over his. At the part where the FBI guy shoots at him from the helicopter and Bruce jumps to a lower level of the roof, you can see the tops of the fake feet that aren't tight around his ankles. (01:50:30)
Continuity mistake: When Maggie starts cleaning up the flour, she hears a noise and looks at the three blue flowerpots, and there is a wooden chair right beside them. Then when it cuts to Maggie getting up, the chair is now about a foot away from the flowerpots. (00:22:50)
Continuity mistake: When Kristen meets the little girl drawing the chalk house, between the two shots before it starts raining, the chalk drawing changes. (00:02:35)
Revealing mistake: When the Blob is eating Paul at the doctors office, his right arm is the only part not covered. It's very obvious that it's in a fixed plastic sleeve surrounded by the Blob's ooze.
Continuity mistake: Near the end when they're entering the Mexican village, Billy's coat disappears as he rides through the gate. (01:17:45)
Revealing mistake: In the barber shop scene at the end of the film, Agent Anderson terrorises Deputy Sheriff Pell (who has a full head of hair) with a razor blade. Anderson throws him across the room, but the person being thrown across the room has a mostly bald head.
Visible crew/equipment: When Dr. Loomis arrives to the gas station the boom mic's shadow is visible on the ground. (00:19:02)
Revealing mistake: During the big fight scene, you can tell easily that Frank is wearing knee pads (try looking when his pant leg is wet). Construction workers will often wear knee pads for work but most often they wear them on the outside of the pants to save the knees of their pants. (00:58:50)
Continuity mistake: When Harrison Ford is on the rooftop and the suitcase accidentally opens, the Statue of Liberty goes flying off the rooftop and on to the ground, several stories down. In the next shot, the little statue is perched at the edge of the rooftop. Also, the clothes that fly away magically reappear next to the suitcase a shot later.