Continuity: There is a scene in which it is raining quite heavily and Em and Richard are inside the hutch. Richard is playing Pop Goes The Weasel over and over which annoys Em. She stabs him in the foot and he chases her outside. In this scene she is wearing a strapless dress but when he catches her and pins her down on the sand she is wearing a lacey jacket (she didn't have time to change) and there are no clouds or rain. The sand doesn't seem to be very wet either, despite the fact that it was raining heavily a few minutes ago.
Great sites
Mistakes
When Richard spots Paddy laying on the ground, they sail over to him. As they run up the beach there's hardly any seaweed around Paddy's body. Yet when it cuts there is much more seaweed around him. Also to the right of the screen on the beach, there are some rocks, but when it cuts (in the same shots as the aforementioned mistake) another large rock appears. See more...
Trivia
Brooke Shields had to stand in a ditch in some parts of the movie because she was much taller than her co-star. See more...
The Blue Lagoon mistakes
Other: This mistake happened right after Richard evicted Emmeline from their hut. We see Emmeline under some rocks and it is raining. See has a makeshift wall made of palm leaves. The rain is coming down to the left, but the wall comes down the opposite way to the right, against the wind and the rain. The director mentioned on the commentary they used ropes to do this.
Continuity: When Richard goes for a swim while Em is inside the hut, you can see him under the water with his hair drenched. He then climbs onto the shore and his hair is completely dry and curly again.
The Blue Lagoon trivia
Brooke Shields had to stand in a ditch in some parts of the movie because she was much taller than her co-star.