Question: why did ezio see altair?
Answer: He saw Altair through some sort of bleeding effect. When Ezio is at the top of the tower on the plank where Altair took the leap of faith in the first game, Ezio looks over at Altair again. When the camera shifts, you can see the templar guy look over and not see anything, and the plank where Ezio saw Altair is void..