This is my guess but I suppose it may happen in a particular scenario where you keep the shutter open in bright sun with no ND filter, and leave this open for 15 minutes. You could literally cook your sensor in such a circumstance, I'd imagine.
The right exposure time to be used is really 15 minutes, I don't think there'd be an issue. I know a lot of overseas photographers who shoot loads of star trails, and some of them keep the shutter open for way longer..