If you notice as the last shark heads for the fence to escape... it "blinks its eyes" Some have noted that sharks don't have eyelids & that this was a "computer mistake". But... if you know enough about sharks... you'll realize that they have something like an eyelid.. whenever it goes for an attack, it's eyes roll back & a film-like covering envelopes the eyes protecting them.

So.. you ask what the slip-up is... when the shark supposedly "blinks" it is not going in for an attack or trying to get through the fence yet.. so there is no need for the "eyelids" to protect the eyes. The shark was merely turning to the scent of the blood. Ooopps....