After the photographer has been bitten by a spider, he lies down on his bunk and dies. He is lying on his left-hand side, dead, when a little blood dribbles out of the left corner of his mouth and runs down an inch or more onto his left cheek. When he is shown in his coffin a few moments later, he still has blood in the left corner of his mouth, but the bloodstain is much smaller and, instead of running across his cheek, it is pointing down - towards his chin.
When a mouse is injected with venom from one of the spiders it is placed in a jar and left on the table top. The camera pans to the doctors so that the mouse is not in shot, and when the camera pans back to the mouse you can clearly see the jar being removed and replaced with a dead mouse in a jar. [This is visible only in the full-frame version shown on broadcast TV -- the proper, letter-boxed, version does not show this at all (it's out of the frame).]
In the shot were the flaming General is hit with the nail from the nail gun, you can see the wire that's attached through the spider and to the nail. Also, in the following scene, the general goes in a straight line to the nest, diagonally from about floor level, across the room to the ceiling, defying the laws of physics.
After the Margaret's party where Doctor Jennings (Daniels) is introduced to village residents there is a shot of Margaret's feet as she enters her house followed by a spider. There is a noticeable 'Wobble' and the camera position shifts slightly as the frame is cut after the actress passes before the spider enters. (The spider would have been filmed separately then the shots spliced together.)
The goof items below may give away important plot points.
After Miller (the football player) dies his helmet is removed by one of the other people on the field and a spider crawls out of the ear hole, then another player steps in closer to the helmet and kills the spider, but after the cut to the wide angle, there is nobody standing that close to the helmet. The only one by the helmet is kneeling.
In the basement when Dr. Jennings throws the spider against the fuse box, the spider is electrically "fried" by contact with the fuses. A fuse box and the outside of the fuses are insulated to prevent exactly what is shown to occur.