Special Effects Failure: The footage of the tarantula (which was a real one) is superimposed over that of sets and landscapes. As such, occasionally, you can see the spider reach the end of the film and fade out or (much more rarely) see through it. Mostly, though, it appears as an almost solid black mass.
And don't forget all the times when the monstrous tarantula roars and growls right before it eats someone.
What an Idiot!: Okay, so the Mad Scientist has a plan to end world hunger by creating giant animals. So he tests his new serum on rabbits, guinea pigs, and a freaking tarantula?
Later on, The Hero finds a mystery substance on the ground. So what does he do? He tastes it.