Errors in geography
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 appears to be a representation of the Texas Department of Criminal Justice, Huntsville Unit. This is due to the notable inclusion of the solid wall surrounding the facility as well as the construction of several of the buildings inside the compound. The Huntsville Unit is also home to the state death chamber and houses death row offenders just prior to their execution. However, the Huntsville Unit (a.k.a. The Walls) is actually constructed of red brick and was opened around 1850 so it an older looking facility. The unit is also located in the center of the downtown area of Huntsville, TX. This city is located approximately 70 miles north of Houston, TX and is home to Sam Houston State University as well as 5 prison facilities located within its city limits.
Incorrectly regarded as goofs
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.
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.