yeah it could be a molding defect since it is on the outside of tank....and weene i guess youre right, there was water when the incident occured and I cant find any cracks/scratches... So its 100% not likely to dent at all ?
as for a "temp" where acrylic could dent.... umm.. i dont have an exact number.. but somewhere just shy of the melting point... so unless you are resting a heatgun on the tank or working on it in an oven.. i dont think you really have to worry about this..
youre right i just spoke to the guy I bought my tank from (Tenecor) and they said the worst that can happen if you drop something to the bottom is you would only scratch the surface. So i guess a dent isnt possible even from a rock, only would scratch it.