Goofs

When Prof. Deemer inspects the animals in his cages, he changes the card (on the clipboard) showing the age of the first animal (white rat) from 5 to 6 days and writes that the rat received the second injection on the 6th day; however, even though he holds the needle in his hand, he does not give the rat an injection. He then inspects the clipboards for the remaining animals yet does not change the date of the age for any of them.

Jacob's presumably dead body changes position, especially his feet.

Early in the film, as Dr. Deemer goes into the lab and puts on his apron, he wears a shirt with short sleeves, but when he puts his arms in the isolation box, the sleeves are long, like a lab coat, then short again in the wide shot when he pulls his arms out.

The jets that take off to attack the tarantula at the end of the movie are F-84 "Thunderjets"; however, the attacking jets are F-80 "Shooting Stars."

As the two officers are running to Matt's convertible, both officers are seen putting their machine guns into the back seat before climbing into the front. But as the camera angle changes to a close-up the officer on the passenger side once again has his gun in his hands as he enters the car.

Dr. Townsend describes the tarantula as belonging to the species Arachnida. Arachnida is in fact a biological Class, not a Species.

Throughout the movie, characters mispronounce the condition acromegaly (it is not pronounced acro-megal-ia)

The movie (and all other "giant monster movies") ignores Galileo's Square-Cube principle. An object's volume (and therefore its weight) increases as the cube of the growth factor, but the strength of legs increases as their cross sectional area (which increases as the square of the growth factor), so the bigger the creature, the weaker the legs are in relation to the weight. The legs would shatter under the weight of the bigger body. However, this is easily explained by the serum's ability to make an organism stronger as well as bigger.

Prof. Deemer takes special care to fill the hypodermic needle inside an isolation box wearing rubber gloves; however, once filled, he pulls it out of the box with his bare hands and then removes air from the needle by shooting a little bit of serum out. Had the toxicity of the serum been that dangerous, he probably would have done that with the needle still in the box and would have worn gloves while handling the needle outside the box.

In one shot of the tarantula crawling over the mountains, the matte is cut incorrectly, so that one of the spider's legs seems to disappear in mid-air.

When Paul attacks Prof. Deemer, in one close-up his "teeth" can be seen flexing, revealing them as being made of rubber like the rest of the mask.

Twice in the film, Dr. Hastings lands his plane on an unpaved desert runway, yet the sound effect when the plane touches down is the screech of tires hitting pavement.

The shadow from the off-screen hand holding the first rock in the "rockslide" can be seen before and after the rock is released.

When the tarantula escapes its cage, the stick used by the handler is plain to see as it moves left and out of frame.

Spiders cannot grow without shedding. In order to get so large Tarantulas would have had to shed her skin many times, leaving a trail of empty exoskeletons behind. It would have also taken her a lot of time since every shedding requires to regenerate a new exoskeleton.

Spiders need to "sting" their prey with their chelicera in order to inject their digesting acid. When Tarantula attacks the farmers her chelicera are already the size of a human, so she could not use human as food.

When Matt Hastings suggests that Josh was listening in on the phone conversation, Josh asks, "Are you inferring...". This is the wrong word. Doc was implying what he had done; Josh was inferring what Doc meant.