Though there's nothing wrong with your stock standard batch of cookies, there is in fact a slight problem with using a highly malleable, heat vulnerable, material as your oven tray. Maybe they thought they could get away with it because they made sure to grease the pan before putting it in? possibly the thought of just leaving it in for a few minutes would do no harm? The likely answer is that the culprit involved has very little, to no idea about the science of melting plastic and the fact that ovens, in fact, create heat on said plastic.