Other mistake: When Al Pacino is supposedly dead, on the left side of his neck ( to our right ) his pulse can still be seen slightly.
Other mistake: When Al Pacino falls into the water off the floating logs, the platform on which he climbs out of the water onto is already wet and even has footprints leading to the metal walkway behind him (which at the end of the scene he leaves on).
Chosen answer: Because he ran out of bullets in his first gun.