Ezio Auditore came across a thief and heard him out. The thief told Ezio about a traitor in the Thieves Guild, who was actually a high-ranking member of the rivaling gang, the Cento Occhi.
Ezio walked up to the thief.
Thief: Ezio, one of the Cento Occhi (Hundred Eyes) leaders has worked his way into our own guild! He is an accomplished liar, responsible for the deaths of many of our brothers. Come, I will lead you to him.
After leading Ezio to a secluded location, the thief signalled the Borgiaguards, and revealed himself to be the traitor. Ezio defeated the opposition and assassinated him.
Ezio killed the traitorous thief, along with the Borgia soldiers that assisted him in the ambush.