Continuity mistake: When Philip surprises Alex and Corrine undressing in the living room, Corrine throws her panties in the air and they land on Philips' head. Philip pulls the panties off his head and throws them down. After a quick cut to the surprised Alex and Corrine, the camera cuts back to Phillip with the panties magically back on his head.
Revealing mistake: When Nick is being sucked out of the hole/cave, you can see the wires visible from his waist. (01:20:35)
Revealing mistake: During the Hy Brasil sinking sequence you can see a horizontal line in the water in the background, this is the edge of the water tank they used to film this sequence. It's also visible in several shots from the longship.
Continuity mistake: When Brian spends his first night in the underground monster world he goes in wearing a shirt and boxer shorts. But a later scene shows him wearing a shirt, pants, and shoes. He then goes back home and is back in his shirt and boxer shorts.