I gather from what I've read that smarts are kinda prone to rolling when hit by a heavier vehicle. The smart is hard like a nut. It activates the other vehicle's crumple zones, but when physics dictate that's not enough to neutralize things, the little booger bounces off and can roll. Wear your seat belts. It is light, and hard. In a serious collision, rolling is likely better than crushing.
There is not evidence I've seen that the smart is prone to rolling in a non-collision situation.