Shimano splines fail

Hi,

I am trying to find out if the splines failing on the crank arm spindle of the 5100 hollowtech is common?

I received a new bike from a main online seller. On my first ride less than 4miles into it, the splines failed I flew off the bike around 15-20mph and removed the skin from my right leg heel to hip including my knee. The right foot is black from bruising when the splines failed I was mid-stroke on the pedal. The seller is going to investigate, its either their lack of tightening bolts or Shimano's fault producing weak parts.