Unusual. The more common points of leakage are from the flapper seal (the nut may not be tight enough), or around the hold-down bolts. It sometimes helps to sand the interior surface to get the rough edges off the casting prior to installing the washer and bolt. Water can migrate, and the location of the leak can be misleading. If the toilet is used frequently, it could also be condensation. Normally, this isn't a problem since the tank is not emptied and the retained water is normally warmed to room temperature, tempering the incoming cold water. But, if you flush the toilet often enough, or your incoming water is excessively cold (say from a deep well), then condensation may be occuring. If it is a fault in the casting, the only reliable solution is replacement. Toto return rates are nominally <1%, which is quite low compared to most other brands. Nothing's perfect all the time. I'd look at the other things first, though.