Goofs

Dr. Stanley leaves the axe at the mansion's entrance, where it falls to the floor. When Dr. West and Dr. Stanley later go back to the mansion, the axe is now neatly propped up against the wall.

The dead silicate found on the beach is bleached white. During its subsequent necropsy, it is the same dark gray color as living silicates.

When the three men land by helicopter on the island, one commences to autopsy the body. Yet earlier, this has already been done.

Peter Cushing enters a house at night shining a torch in front of him yet he casts a shadow.

Dr. Stanley shows Dr. West a drawing of what he says is a protein structure, then says, "It's not adenosine triphosphate." Adenosine triphosphate (ATP) is a nucleotide, not a protein.

When the villagers are throwing sticks of dynamite at the creatures, the explosions given off by the dynamite are no way near as powerful as would come from real dynamite blowing up.

Dr. Phillip's dog is hit by a strong dose of Strontium-90. Later, strontium 90 is taken from Phillips' reactor to dose the cattle. In 1966, the nuclear reactors capable of producing Strontium-90 were far larger than the one in Phillips' lab and needed giant control boards, as well as huge amounts of water for cooling, none of which were in evidence. But it's never mentioned that Dr. Phillips manufactured the Strontium 90, only that it was stored in his laboratory.

During the silicates' attack on the main building, the townspeople flee into a back room, with many trapped outside in the hall screaming to be let in. Seconds later, a silicate tentacle smashes through a door panel, implying that those who screamed to be let in were killed; yet later, when those within the back room open a second set of doors to see the silicates dead, not one single human corpse is present. That's because there's a hallway between the main hall and the back room that remained empty of people.

After Dr. Landers is killed, his corpse mysteriously vanishes -- well, up until it's spotted on the floor of the cellar where he was killed.

When the townspeople attack the silicates in the woods with dynamite, the explosions reveal the wheels propelling the creatures along.

At around 29 minutes, the farmer goes to the police house to report that his horse has been killed. When Constable Harris opens the front door, we can see a wall just behind the farmer; however, at around 33 minutes, Dr Landers pulls up outside the same door in his car on a yard or road with no wall anywhere in sight, thus making it obvious that this is a location shot and the earlier a studio shot.

The cast list at the start of the film does not match up perfectly with the list at the end. The names of actors Carole Gray and Eddie Byrne appear, respectively, third and fourth at the start, but fourth and third, respectively, at the end. The names of actors Liam Gaffney and Roger Heathcote (as they are correctly spelled at the end) are spelled as Liam Caffney and Roger Heathcott at the start. The name of actress Margaret Lacey (misspelled as Margaret Lacy) only appears in the opening credits - the only name not to appear in the end credits as well.

The entire population of the village is sheltering in a building, but there are no children. Village of grownups only.

The island is on the "east coast of Ireland". There are very few and none large enough for such a village. The west coast has many inhabited islands, however.

When Dr.s Stanley, West and Landers initially come to Dr. Phillips' mansion, no one comes to the locked front door when they ring, forcing Dr. Stanley to pry his way inside through an unlocked side window. Entering the library through a closed door, he finds a human corpse, stepping past it to let the others in. Since the tortoise-like creatures are later seen to glide slowly over the ground with manifest strength to break glass and punch through wood with only one tentacle to aid them, it's unexplained how they got out of the lab (or the library) leaving no door, window or wall broken. There is also nothing to suggest -- being eating machines with a single-cell disposition -- that they'd be either cleaver, considerate or facile enough to close doors behind them when finished eating. How they escaped the mansion to roam the island remains inexplicable.

An entire room is dedicated to test animals, yet none are seen. Every cage is empty. The silicates would likely have feasted on them but would have attacked and mangled the cage bars to do so, leaving flesh and fur behind, but the bars are pristine and there are no animal remains.

The silicates are later shown not to be fast movers, yet when one slides down the back window of a car, it disappears completely by the time men run over to see what happened.

After seeing the creature on the back window of the car she's in, Toni pleads with the three men to join them, saying "I'm not staying here with all those things running around." She saw one creature and has no reason to believe there would be more than one.

It is never shown or explained how the silicates are able to climb trees or get onto roofs.

When the silicate slides down the back of the car and thereafter seems to hide from the woman's scream and the approaching men rushing over to investigate, it doesn't need to. Later shown to be impervious to axes, bullets, fire bombs and dynamite, men pose no threat. Hiding, with the suggestion of laying in wait to catch a human meal off-guard (or fleeing for protection), defies the single-cell mentality of this unstoppable eating machine.

After cutting off Brian's left hand, David puts him in the back seat of the car to wrap the stump. While doing this, Peter Cushing lifted his left hand up into view.

Throughout, vehicles are left alone outside with open windows and characters leap into them without looking first.