Untruth

Drona firmly believed that Yudhishthira would never speak an untruth even for the sake of the sovereignty of the three worlds. For this reason, that bull among Brahmanas asked Yudhishthira and not any body else. He had hoped for truth from Yudhishthira from the latter's infancy.

"Meanwhile, O monarch, Govinda, knowing that Drona, that foremost of warriors, was capable of sweeping all the Pandavas off the face of the earth, became much distressed. Addressing Yudhishthira he said,

'If Drona fighteth, filled with rage, for even half-a-day, I tell thee truly, thy army will then be annihilated. Save us, then, from Drona. under such circumstances, falsehood is better than truth.

1. By telling an untruth for saving a life, one is not touched by sin. 
2. There is no sin in untruth spoken unto women, or in marriages, or for saving king, or for rescuing a Brahmana.' 

While Govinda and Yudhishthira were thus talking with each other, Bhimasena (addressing the king) said, 

'As soon, O monarch, as I heard of the means by which the high-souled Drona might be slain, putting forth my prowess in battle, I immediately slew a mighty elephant, like unto the elephant of Sakra himself, belonging to Indravarman, the chief of the Malavas, who was standing within thy army. I then went to Drona and told him, 'Aswatthaman has been slain, O Brahmana! Cease, then, to fight. Verily, O bull among men, the preceptor did not believe in the truth of words. Desirous of victory as thou art, accept the advice of Govinda. Tell Drona, O King, that the son of Saradwat's daughter is no more. Told by thee, that bull among Brahmanas will never fight. Thou, O ruler of men, art reputed to be truthful in the three worlds.' 

1. Hearing those words of Bhima and 
2. induced by the counsels of Krishna, and 
3. owing also to the inevitability of destiny, 

O monarch, Yudhishthira made up his mind to say what he desired. Fearing to utter an untruth, but earnestly desirous of victory, Yudhishthira distinctly said that Aswatthaman was dead, adding indistinctly the world elephant (after the name),

Before this, Yudhishthira's car had stayed at a height of four fingers' breadth from the surface of the earth; after, however, he had said that untruth, his (vehicle and) animals touched the earth.

http://www.sacred-texts.com/hin/m07/m07187.htm

Yudhistira to his Army - When Narayana Astra Was Released By Ashwatthama

Let the wishes of king Duryodhana be crowned with success today, for I have today slain in battle the preceptor, that always cherished such friendly feelings towards us,

1. that preceptor, who, without protecting, caused that child unacquainted with battle, viz., the son of Subhadra, to be slain by a multitude of wicked warriors, 
2. that preceptor, who with his son, sat indifferently, without answering, when Krishna in such distress, dragged into the assembly and sought to be made a slave, asked him to say the truth, 
3. that preceptor, who, white all the other warriors were fatigued, cased Duryodhana in invulnerable armour when the latter desired to slay Phalguna and who, having cased him so, appointed him to protect Jayadratha, who, being acquainted with the Brahma weapon, scrupled not to exterminate the Panchalas, headed by Satyajit, that had exerted themselves for my victory, 
4. that preceptor, who, whilst we were being unrighteously exiled from our kingdom, freely told us to go into the woods although he had been solicited by our friends to withhold his permission.  Alas, that great friend of our hath been slain! .

For his sake, I will, with my friends, lay down my life

http://www.sacred-texts.com/hin/m07/m07196.htm