1992 Toyota Camry oil pan/change

I took my car in to the dealer for a routine oil change. What attracted me to this dealership was it was a brand new location, they guaranteed work in thirty minutes and a cost of $12.95! Lo and behold at the end of the "long" wait I was taken to the car and shown the threads of the oil pan bolt had come off and the only solution was to replace the whole pan at a total cost in the $500.00 range! If the previous oil change was done at the same dealership (at their old location) and the next one at the new location, how in the world could the threads have come off? Is this normal?