I don't think this could happen without something jamming the impeller, it is marginal as to whether this is a warranty issue. The back half is protected by that black strainer plate but something would have had to have stopped the impeller to strip the spline. If you are in the US, I will work with you to get you a replacement, these impellers are very expensive. If you are outside the US let me know and I can refer you to the correct service center.