The Slaying Of Dronacharya

The great guru had just one weakness and that was his son Ashwatthama, whom he loved dearly. So Lord Krishna suggested that they somehow manage to convince him that his son was dead. Once he was unnerved it would only be a matter of time before he was out of the running. Neither Arjuna nor Yudhisthir were happy about this and only agreed to go through with this after realising that the battle would otherwise be a lost cause. They then decided to kill an elephant by the same name and then go to Dronacharya with the news so they would not be lying.
Bhima, the second and strongest of the brothers went ahead and killed an elephant by the name of Ashwatthama and then went up to Dronacharya and said, "Ashwatthama is dead." Dronacharya was shaken to the core of his being and was absolutely heartbroken at first. And then on second thought he realised that his son was almost as strong, skilled and powerful as he was and
could not easily be defeated. He also knew that Yudhisthir was an extremely truthful man and so he decided to ask him if Ashwatthama was really dead.

Yudhisthir, knowing that everything depended on his word, truthfully said that Ashwatthama was dead, as the elephant really was dead. Dronacharya fainted. He recovered enough to fight once again but his spirit was broken and he was slain in battle that day.

Legend has it that as Yudhisthir was so morally upright, his chariot always travelled two inches above the earth, but after that day his wheels touched the ground. He might not have said an outright lie but it was a white lie nevertheless, even though it was for the greater good. So according to the laws of the universe he had to pay for it.