Spontaneous failures in thermally toughened glass are frequently reported, although the actual percentage is quite small, suppliers in the Netherlands reporting one or two cases a year while thousands of tempered panels are supplied. These cases are rarely investigated in the Netherlands because the broken thermally toughened glass is usually not in one piece and can thus not be sent for analysis. In 2015 several sliding thermally toughened glass single curved vehicle roof windows failed within several months. A butterfly pattern was seen and NiS failure was suspected by the window manufacturer. As a coloured adhesive foil had been applied during manufacturing the glass shards were kept together. The broken glass was shipped back to the supplier and was together with several non-broken roof windows sent to Delft University of Technology for analysis. The compressive surface pre-stress in the intact roof panels was measured with a SCALP 5 device. It was found that the compressive surface pre-stress varied, as bands of low compressive surface pre-stress alternating with bands of high compressive surface pre-stress appeared in the specimens. Destructive testing of the intact windows showed that the windows failed in the bands of low compressive surface pre-stress. The actual bending strength was far below the strength thermally toughened glass should have. Microscopic analysis of the butterfly fragment of the “spontaneously” failed roof windows showed that the failure was not caused by any inclusion in the glass, but started at the surface of the glass and was probably due to overloading. It is concluded that the failures were not spontaneous but the result of overloading due to uneven tempering of the glass.