When Meg gets out of her car to talk to Brian as he's walking down the street at night, the head of a boom Mic is reflected on the window of the china shop at the far left of the screen just before the camera stops moving backwards
When Candy Clark is bringing dishes back into the diner kitchen after trying to unclog the sink and she sees the cook being dragged into the sink, the crew can be seen in the reflection on the swinging door's window.
Because of the highly corrosive acids The Blob uses to dissolve its prey, there should be some kind of trail of acids it leaves behind where ever it goes. But not so much as a drop of it is shown throughout the movie. Instead, it seems to leave no sign of it in its destructive wake.
The projectionist at the theater calls another employee and tells him that the projector booth is very hot even though the AC is at full blast. As he waits for his coworker, he takes a look in the vent and gets killed by the blob. This would be impossible as extreme cold is the blobs major weakness so there would be no way for it to even get into the vents or even get into the projection booth with the AC running.