I believe that's an urban legend along with the one about the cyst in the chicken burger. Not least since the grills in McD's are on timers so that the thin patties cook in exactly 35 seconds and the cooking staff don't have any authority to make judgement calls.

I worked briefly in McDonalds while at University. My overriding perception of the place was that in terms of rules and regulations about health and safety, it was top notch. Obviously enforcing those rules depend on the managers on the ground obeying those regulations but I can honestly say that if I had to eat in a fastfood place, I would be more confident in McD's than most others.