Visible crew/equipment: When the girls light the first flare in the cave, the top of a crew member's head is visible, as he is crouched behind a rock. This is confirmed by Director Neil Marshall on the DVD cast commentary.
Continuity mistake: Right near the end of the film, one of the characters is driving away from the caves and eventually pulls over onto a grass verge where she breaks down in tears. Seconds later, a lorry carrying logs drives past her sounding the horn. At this point, you can see that her vehicle is now on the road.
Continuity mistake: When the girls are crossing the first chasm over the huge drop, the first two girls make it to the other end safely using a wire. When the two girls are hooking up the third girl to come across, we can see four people stood behind her. There should only be three.
Revealing mistake: Towards the end when Sarah and Juno team up to find the exit, they pass underneath some thin stalactites. As Sarah goes under them, one of them wobbles as if it is made of rubber. In reality stalactites are rigid formations and are not supposed to wobble when touched. (01:26:40)
Revealing mistake: When one of the girls falls and tries to get up to a tunnel, she grabs a large rock to help herself up. You can see the rock bend and wiggle under her weight.