Originally Posted by cny-bikeman

Mistake one is thinking a bike you are not familiar with is "good to go" without checking it over thoroughly.

Perhaps you did not tighten the q/r, but it's unlikely the bike shop person also tightened the q/r improperly. It IS possible that he did not check the axle or q/r springs. If the axle sticks out on either side of the wheel further than the dropout is wide then the wheel will not clamp properly. Likewise if the narrow part of either q/r spring is not pointed toward the inside/axle rather than out toward the lever.