Failure analysis of tempered glass windows is normally impossible because the failed window falls apart into a large number of similar pieces, and locating the origin is impossible. However, there are cases when the window remains intact, and root cause failure analysis is possible. In this case the glass expert was able to preserve the origin and perform the failure analysis. Three failed windows were analyzed. In all cases the cause of failures were caused by a nickel sulfide contaminant particle (NiS). In one case, the NiS particle was visible in a macro-photograph. This is seen in the top left photograph. The other two images are photomicrographs of the NiS particle as it appears on the fracture surface at the origin. it is rare to be able to see the particle with the naked eye, but this is an exaple of such an event. The composition of the particle was determined to be Ni and S using energy dispersion spectroscopy. Failures of this nature are well understood and documented