I did a google search on BMW water pump pulley failure. I got lots of hits on water pump failures, but the number one search return on a pulley failure was for an aftermarket pulley, made by a company often discussed on these forums. It failed due to fatigue. Can you imagine that, metal being subject to fatigue?

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue