Revealing mistake: When the scientists inject the mouse with poison, they put it in a jar. The, after they place it down you can see someone's hand swap the jar for one containing a dead mouse. You can even hear it. (01:17:58)
Revealing mistake: When Becky sees the spider in the shower, there are two instances when you can see the bodystocking she's wearing, covering her breasts, to preserve her modesty, most noticeable when she screams. (01:11:53)
Revealing mistake: When Jerry Manley gets bitten by the spider in the tent in South America, the others come to look at his corpse, you can see clearly see his neck pulsating, when he should be dead. (00:15:35)
Revealing mistake: At the end when the dock 'kicks' the king spider off him into the fire, we hear it scream like a stuck pig. Then additional screams from the 'burning' spiders inside the pulsating nest, and when the doc torches both the king spider afterwards.
Revealing mistake: When the flaming "general" spider is killed, there's several mistakes. When it leaps, it suddenly "freezes" in mid-air for a second before the nail hits it. Then, when we see the nail hit it, you can see a wire attached to it, plain as day, to yank it back. Then, when it hits the egg-sack, you can see several fake spiders attached to it, which don't move at all. Then the egg sack catches on fire, but the flames come from behind the sack, even though the general hit the front.