I’ve seen it many times when working with novices: A loose object in the car–loose change, tube of sunblock, whatever–often leads to a momentary lapse in concentration.
In reply to David S. Wallens :
I can’t seem to get rid of the loose nut behind the wheel. 
This is why my race bucket has a massage function.
When I was just starting out, the passenger seat belt buckle used to bang against the B-pillar through slaloms and distract me. So, before my runs, I’d buckle that belt, too.