Continuity mistake: When Mac's on the windshield and the car starts to dodge cars, one shot from inside the car shows the alien has disappeared.
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: 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)
Visible crew/equipment: When the Critters are in the restaurant, The Hungry Heffer, as the camera pans across some Critters eating lettuce and other food. If you look closely, you can see one of the puppeteers hands coming out of the bottom of a critter puppet. The puppeteer is wearing a dark shirt with what look like gloves.