Continuity mistake: When Maggie starts cleaning up the flour, she hears a noise and looks at the three blue flowerpots, and there is a wooden chair right beside them. Then when it cuts to Maggie getting up, the chair is now about a foot away from the flowerpots. (00:22:50)
Continuity mistake: When Kristen meets the little girl drawing the chalk house, between the two shots before it starts raining, the chalk drawing changes. (00:02:35)
Revealing mistake: When the Blob is eating Paul at the doctors office, his right arm is the only part not covered. It's very obvious that it's in a fixed plastic sleeve surrounded by the Blob's ooze.
Visible crew/equipment: When Dr. Loomis arrives to the gas station the boom mic's shadow is visible on the ground. (00:19:02)
Continuity mistake: Throughout the film Elvira wears her trademark black dress, split to the top of her thigh, and underneath it she always wears black pantyhose. Inexplicably, this changes to black stockings and suspenders when she is undressing for bed on her first night in her new house. (00:31:35)
Revealing mistake: During the big fight scene, you can tell easily that Frank is wearing knee pads (try looking when his pant leg is wet). Construction workers will often wear knee pads for work but most often they wear them on the outside of the pants to save the knees of their pants. (00:58:50)
Continuity mistake: When Bill Pullman is poisoned he has a defined red cross painted on his forehead. He stumbles around for a while and the cross smears to the point where it looks like a big red blob. But when he finally dies and looks at the camera, the cross isn't smeared at all. (01:16:35)