The practice of severing the ring finger for assassin initiates was discontinued by Altair, the master of the Brotherhood, sometime after its reformation in 1192. This was referenced in the video game Assassin's Creed 2, occurring decades before the events in the movie. As the movie and its sequels are original stories set within the same universe as the games, removing a finger to use the hidden blade has not been necessary for centuries by the time Aguilar joins the Brotherhood. Ubisoft and the Assassin's Creed franchise team explained that the Spanish branch of the Brotherhood to which Aguilar belongs still severs one finger per initiate to ensure the commitment and loyalty of each initiate to defend both the Brotherhood and the Creed. This is why Aguilar has the ring finger severed on only one hand despite wielding two hidden blades; the severance is no longer about function, but rather about symbolism and dedication.
In the opening scenes Young Cal has several scrapes on his face after crashing his bike, which disappear when he finds his mother dead and then reappear in the next scene when he is running across the roofs.
The Assassins enter the Templar gathering by passing through a metal detector without setting it off. Once inside, one Assassin removed a blade from his mouth. The blade should have been detected by the detector regardless if it was hidden inside the Assassin's body; but we see the Assassin point at his belt during safety checks, assuming the guard to believe it was the belt that had the detector to go off.
The movie takes place in 2016. It is mentioned during the movie. The opening scenes the action takes place 30 years earlier, but the cars in those scenes look more modern than 1986 models. For example, the SUV shown prominently is a 1997 Mercedes-Benz G-Class.
When Cal is in the Animus towards the end of the movie, Sofia asks where he is. The lab technician replies that it looks like a military port and adds "Cádiz, Palos de la Frontera". Palos de la Frontera is in Huelva (also in Andalusia), not in Cádiz.
In the opening sequences exploring Cal's background before being taken to Abstergo - Madrid, (time code- 8.34, text reads "30 years later") the text that reads "Huntsville Department of Criminal Justice, Texas USA" is incorrect. This is not the geographical location for Huntsville, Texas. The Prison facility, most likely a representation of the Texas Department of Criminal Justice, Huntsville Maximum Security Penitentiary (a.k.a. The Walls unit) is actually looking aged in the center of the downtown area of Huntsville, TX. This city is located near Houston, TX, approximately 30 minutes away.
The real Knights Templar were condemned as heretics and disbanded in the 14th century.
The real Assassins were an Islamic creed, destroyed by the Mongols in the 13th century.