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.
Continuity mistake: When poor George is about to be sucked down the drain by the blob, he is looking at his hand. When the blob attacks him, he's holding on to the sink with both hands.
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.