When Mike and Carol are exchanging notes, Carol takes the paper and can clearly be seen to write three separate lines of text (ie: she returns to the left-hand margin and begins writing three times), yet when Mike looks at what she's written, it is shown as only two lines.
The spider awakens and scares the kids at the band practice in the school. The drummer looks up in horror and flees his drum kit. In the following wide shot , the drummer is back to playing his drums again.
Professor Kingman tells the Sheriff that, even dead, the spider needs to be studied so that scientists can learn why it grew so large and so prevent a race of giant spiders from enslaving Earth...a supposedly key plot element is never mentioned or dealt with again.
The so-called teenagers look more like post-graduate students than high schoolers. Skip Young, who was also a regular on "The Adventures of Ozzie and Harriet," was 28 and Tony Patterson, who played "Joe," was 35 years old.
When the spider is walking down Maple Street to the Kingman's house, you can tell that the set is a miniature. The houses are all tilted to the right and there is a big shadow in the foreground, which is out of focus. In the shot of the real street seconds before, all of the houses are straight, the foreground is in focus, and there is no shadow.
As the Sheriff and Professor Kingman watch the spider walk past the Sheriff's Office, they keep turning further away from the spider until they're looking directly ahead while the spider is off to their right.
Throughout the movie, most split scenes simultaneously showing the spider on a miniature set and humans on a real set (street scenes, in the cavern, the high school auditorium, etc.) are badly done and obvious, with different lighting, mismatched sets and a large dark border marking the separation between the miniature footage and the full-sized film.
When the exterminators are shown spraying DDT at the spider in another split-scene shot, the men are on the left and the spider at the right, with the two scenes separated by a painting of a stalagmite. But the heavy spray of DDT stops at the split screen, though in real life the spray would have been seen shooting past the stalagmite into the right-hand portion of the scene.