When the tribe is barricaded in a cave, three of them go looking for another way out. The have to swim through an underground pool and come out covered with leeches. When they return and swim through the same pool, they don't pick up any leeches.
Barely surviving in a stone age culture explains why all the men have beards, yet somehow all the women have shaved armpits and legs.
In the beginning, when they stab the monster, there is no blood coming out of the beast or on the spears that stab it.