Reuven is driving behind Shimon and smashes into Shimon's car, destroying his fender and causing $500 worth of damage. Shimon continues to drive home and gets into a terrible accident that completely totals his car. Reuven claims that he should not have to pay for the damage that he caused because even had he not caused the damage, it would have happened due to the later accident. Is Reuven obligated to pay the $500?
Rashash learns from Tosafos that Reuven is responsible.
Tosafos says that although in the case of the gemara where one throws a vessel from the roof and the other one smashes it before it hits the ground, the thrower is chayev and the smasher is exempt; in a case where one throws a stone at a kli and before it hits someone else smashes it, the smasher is chayev and therefore the thrower is patur. The argument of "מנא תבירא תבר" - I broke an already broken item, only applies to a case where the item itself that was eventually damages was thrown, not when another items is thrown at it. The logic is clear. When one throws a stone at a kli, he has no connection to the kli until his stone makes contact with it, so if prior to that someone else smashes the kli, the smasher is fully responsible.
Based on this the Rashash says that if Reuven smashes Shimon's vessel, but later a fire occurs and burns the broken vessel, Reuven is responsible to pay for the damage he caused. Why? Because even if Reuven would smash the vessel after the stone has been thrown, he would be obligated to pay, certainly if he broke the kli prior to the fire heading to Shimon's home, Reuven is chayev to pay. Similarly in the case of the car accident, Reuven would be obligated to pay because even if at the time of the fender bender there was already a train headed right at Shimon's car and it would definitely be destroyed, Reuven is liable for the damage he caused [the rashash in the end suggests that if the fire was already in the chatzer, perhaps reuven would not be liable, but based on this Tosafos he still should be].