Dronacharya was the guru of the Pandavas and kauravas. Guru Dronacharya had taught them all about warfare and some other things. The great guru had just one weakness that was his son Ashwatthama, whom he loved dearly. So lord Krishna decided that somehow manage to convince him that his son was dead. Neither Arjuna nor yudhisthir were happy about this. Then they decided to kill an elephant with the same name and after that go to Dronacharya with the news so they would not be lying.
Bhima, the second and the strongest of all brothers and killed an elephant with the name of Ashwatthama and said to guru Dronacharya that “Ashwatthama is dead”. But he realized that his son was strong, skilled and powerful, so he could not be easily defeated. But he knew that yudhisthir was a truthful man so he decided to ask him that ashwatthama was really dead.
Yudhisthir knowing that everything is depended on his words but he truthfully said that Ashwatthama was dead. Guru Dronacharya fainted. His spirit was broken and he was slain in battle that day.
Yudhisthir 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. So according to the laws of the universe he had to pay for it.