It happens much more often with the larger wheels most cars have today. It doesn't take much deflection to roll a 2 inch sidewall.
Not with moderate under-inflation (3-5 PSI). For that type of deflection to occur, again, the tire would have to be borderline "flat", not just under-inflated and would have to impact a pot hole or a speed bump. Low profile tires have stronger side walls to prevent the scenario you speak of, which is a reason to go with the tire manufacturer's PSI recommendation.
this has now entered ridiculously small point argument but i will just say that most tires that are under inflated are likely 10+ pounds. I don't want to extend my semantic warrior crown so lets not quibble about flat vs under inflated.
