Deliberate mistake: When Lucius is shown Maximus' tomb, the painting on the wall reads, "What we do in life echoes in eternity," in plain perfect modern English language. Of course, it helps audiences relate to the first movie, but it's ludicrous, to say the least.
Deliberate mistake: It's very likely deliberate, but at the end of the movie, Lorne and Rosie are asked by the production which last name Rosie should be credited as, "Michaels" or "Shuster." They pick Shuster. In the actual show, she is credited as Rosie Michaels. (01:37:50)