It happens
I remember selling a 2004 Ultra to a friend brand new when I worked at a dealership. We had ridden the bike during PDI as we always did putting about 6 miles in addition to the 2 it had upon arrival.
The salesman and I had just went over the operation of the controls and my friend jumps on to ride home, lets out the clutch normally and the bike moved maybe a foot and bam...belt broke.
Luckily his wife hadn't left yet with the car....
The belt looked like it was cut with a knife...
All I could say was sorry, we'll get it on the lift and put on a new one ASAP. Not the way you want to start out with a new bike but as far as I know that's the only problem he ever had with that bike.