Factual error: Steve McQueen remarks that the chunk of meteorite is hot - though this is a common misconception (and probably belongs on several movies' boards), meteorites are almost invariably cold, given that by the time they hit Earth their frigid inner layers are exposed by the friction of air drag.
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The Blob mistakes
Visible crew/equipment: In the beginning of the movie (about 10 mins. in) Steve McQueen and Aneta Corsaut are driving along and think they hit an old man on the road. They stop the car and get out. There is a shot of the rear right panel of the car, two sets of "lights" are visible reflecting off the car. One single light and 4 lights in a square shape to the right of it. They help the man into their car and there is another side shot of the car and the lights are now 3 sets of 2 lights. They're in the middle of nowhere, at night, along a country road. It looks like "light trees" used to illuminate a scene at night.
Other: In the scene where everyone is running out of the theater most are cracking up.