Xanatos did pretty much fail his Jedi trials (a mission to Telos with his master Qui-Gon) and after this he became a Dark Jedi. So he wasn't one of the Lost Twenty because those people are said to serve ideals that didn't fit the ideology of the Jedi Order and therefore left the Order. That's hardly true for Xanatos.
Originally posted by Darth_Glentract
Xanatos is not part of the lost twenty. First, the twenty were only Jedi Master's who left, second, there were at least three major schisms in the Order. Are you going to tell me that only 20 of those people left in all three of those?
What are those three major schisms?
Which is much different than storming out and going to the Dark Side. It was more like Dooku said "Jedi Council, I'm giving up my rank in the Order because..." rather than "Jedi Council, I'm going to kill you all! Death! DEATH!"
So, basically, 20 Dookus.
And the three major schisms? The first was when a Jedi named Xendor fell to the Dark Side and took many Knights with him. Another was during the Clone Wars; Jedi who did not want to fight a war, or fell to the Dark Side.
The first was c. 25,000 B.B.Y. led by Xendor. They were defeated by the Jedi.
Then there was the Second Great Schism that led to the Hundred Year Darkness. This was what led to the creation of the Ancient Sith Empire, for it was these Dark Jedi that first met the Sith.
Then there was another Great Schism that led to the New Sith Wars.
The Schism during the Clone Wars was not big enough to be referred to as a Great Schism because after Mace interferred, only a few Jedi fell to the Dark Side, in the Great Schisms, hundreds or thousands fell at one time.