Revealing mistake: In the last scene of the film where the Reverend is holding up the glass jar with the mini-blob in it, you can see the mechanism being used to move the blob (look between the Reverend's fingers). (01:30:30)
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.
Revealing mistake: When Vicki is comatose while The Blob is inside her, you can see her blink despite her already being presumably dead.
Revealing mistake: The scene where Meg fights off the blob using a fire extinguisher isn't composited well. The blob and the street light are translucent. The street light even disappears when it's knocked down.