Continuity mistake: After Meg and Kevin climb the sewer pipe that leads to the street, Meg is unable to fit in the grate. In one shot she's trying to fit her head through the grate but in the next shot she is not. (01:12:22)
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.