Some guys get too excited I guess. As long as they are prepared to pay for their own actions and not blame the PH I guess.

Sometimes the animal is too small as mentioned, or another animal is behind and a complete penetration might hit another animal.

Good advice is to wait for the all clear to shoot.

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JPK is right too. I have had a PH complain about why I didn't shoot at a record klipspringer. The reason - a tree trunk blocked its chest and all I could see was its head and rear end. He then said I should have shot it in the rear end! Sometimes a client can't win.