So my theory is if I use Telekinetic Projectile to throw a Thunderstone at a target it will do the damage as stated in Telekinetic Projectile only. But then I wondered what happens to the Thunderstone after it hits the target the spell ends? So if the target is standing on any hard surface like a stone floor or any other hard surface wouldn't the stone fall to the ground after the effect of the spell which would then trigger the effect of Thunderstone when it hit the ground which would be right after the end of the turn/action?
This would be 1d10 damage to start during the turn and once the turn ends the location of the stone would be 1d4 additional damage.
this idea grew from a goblin NPC who placed a gallon bucket of thunderstone on top of a partially open door for the pc's in one of our games.