If it is truly a tempered glass it won't dent. The glass is designed to fracture into a 1000 little pieces so you don't get cut in half!! Obviously it was manufactured with the defect. When they temper glass it is heated to extreme heat thus making it tempered. It was probably a defect in the glass when heated and it got worse. I have seen it before in side glass in automobiles. They should have no problem replacing the glass if they know what they are looking at!! Also there should be a tempered emblem in the lower corner of the glass. Good luck ,Bill