Ser Osmund Kettleblack, a vision of chivalry in immaculate scale armor and white wool cloak, swore that King Joffrey had long known that his uncle Tyrion meant to murder him. "It was the day they gave me the white cloak, my lords," he told the judges. "That brave boy said to me 'Good Ser Osmund, guard me well, for my uncle loves me not. He means to be king in my place.' "
That was more than Tyrion could stomach. "Liar!" He took two steps forward before the gold cloaks dragged him back.
But is Ser Osmund lying, or is GRRM misdirecting us here? Tyrion certainly can't know firsthand, as even Joffrey wouldn't be stupid enough to say this in Tyrion's presence. Who else is lying at this trial? Everybody seems to be telling the truth except Taena Merryweather and Shae, and both of them have ulterior motives to lie: Taena wants to ingratiate herself with Cersei, and Shae not only has a deal with Cersei, but also wants to dissociate herself from the traitor Tyrion.
Could Osmund Kettleblack be lying here? Probably not. The witnesses have to swear an oath to speak the truth:
and here is what Osmund's brother Osney said about lying to the High Septon in A Feast for Crows:
"Lord Hand," [Ser Addam] began, after the High Septon swore him to speak only truth.
"I never lied to no High Septon before. I think you go to some hell for that. One o' the bad ones."
Cersei managed to persuade Osney to lie by sleeping with him, but is Osmund's testimony at Tyrion's trial worth that kind of persuasion? He just reports what Joffrey thought, and doesn't really tell us anything about Tyrion's actions.
So Joffrey had the motive (he thought Tyrion was planning to kill him) and he had the means and the opportunity (Mandon Moore would have obeyed him). It's not clear who else Mandon Moore would have obeyed apart from Cersei, and not only is she smart enough to realize that Tyrion is helping defend King's Landing, but she also thought she had Tyrion under control because she had his whore captive.