Arachnophobia (1990)

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: When Becky gets out of the shower naked, her father walks in on her and you can clearly see her wearing pants, reflected in the mirror, to keep her modesty. (01:10:05)

Hamster

Revealing mistake: In the scene where Dr. Ross Jennings is trapped in the corner of the cellar, you can see that the tarantula which starts climbing up his leg has a huge black stick stuck to the back of its body, pushing it along. (01:36:05)