Such close quarters, however, makes it extremely difficult for a driver to avoid an incident as it is unfolding in front of them, and the slightest mistake can lead to a multi-car accident dubbed "the Big One" by fans and drivers. It is uncommon, but possible, to see 20 or more cars collected in the crashes. Occasionally, cars go airborne and barrel-roll or slide on their roofs, although NASCAR has made several advances in safety over the years to lessen the chance of a car going airborne.