The Legend of Zorro
I'll go ahead and warn that the following contains some minor spoilers, but nothing too significant. Besides, I wouldn't really recommend the movie (just rewatch The Mask of Zorro), so I'm not too concerned about spoiling it.
In this sequel, Zorro goes up against an Illuminati-esque organization which is intent on throwing the United States into turmoil. Their plan involves providing the Confederate Army with a large supply of nitroglycerin, which will distribute it among the troops and utilize the explosive in an attack on Washington D.C.
As stated in the movie's opening scene, the film is set in 1850. We can even pinpoint the exact date of the film's climax as September 9, 1850, the date California joined the Union. The error here was noticed by many a moviegoer: there was no Confederacy in 1850, nor would there be for another decade.
Even if one ignores all the references to the Confederacy and imagines this was only exploiting America's conflict over slavery, it still doesn't work. Maryland was a slave state in 1850, and the District of Columbia didn't abolish slavery until 1862. Attacking a slaveholding city located between two slaveholding states would be an odd way to strike a blow for other slave states. This would've been concurrent with the Compromise of 1850, which temporarily lessened America's conflict over slavery, making it an even odder time for the South to revolt.
Also, when Zorro first stumbles upon evidence of the villains' scheme, he finds a map of the United States, which the camera focuses on. Strangely, the map looks rather modern, showing all of the present-day boundaries for U.S. states. A map of the United States circa 1850 would have several territories, and should instead look like this:
There are plenty of smaller anachronisms, but these two were the ones that struck me as egregious. It would be like telling a story about the American Revolution in the 1760s, or the U.S. fighting Nazis in 1930. Given their obviousness, one wonders how no one involved in the production managed to catch the errors. And yet the villains' entire plot is based around this massive historial blunder. It's not a good sign of quality writing.