I haven't had a tank failure, but have heard mostly of side pannels getting cracked or silicone on old tanks getting weak. In the couple of cases where the bottom has cracked it was due to something real heavy being dropped on it or the aquarium not being level with the stand or installed on something not appropriate for an aquarium. If installed correctly and given proper care, the likely hood of the bottom cracking is pretty low. I hope this helps.
Chris R.