Factual error: At the end when the oil in the cavern is burning there is no sign of smoke. With a fire as large as it appears to be, everyone in there should be choking to death, yet they don't even seem the least bit bothered. Forest Whitaker should have also been roasting over the flames, yet he is unscathed.
Factual error: At one point in the film, Morgan calls out "Riflemen, to me!". The film is set in the late 17th century. Whilst rifling had been invented prior to this date, it was still rarely used except on the most expensive weapons, and even then mostly used by specialists. It is highly unlikely that all, if any, of the pirates would have been armed with them, especially in the close quarter fighting they would have been expecting. The pirates would be more likely to be armed with the more common smooth bored muskets of the period, so "Musketmen, to me!" would be much more likely to be correct.
Factual error: In the movie, Wild Bill has a shootout with the bad guys in the stables. He is holding two colt revolvers, each having the capability of holding six rounds each. However as a safety precaution, only five rounds were loaded in the revolver with the hammer resting on an empty chamber as a safety. Therefore, he could only have ten rounds total from the two colt revolvers. He fired far more than ten rounds without reloading.
Factual error: There is absolutely no way that Internal Affairs would have any authority or power to shut down an entire police force because of stolen drugs. As in the plot of the film, that type of decision would be decided by the city council on a vote and approved by the mayor. In any case, an extensive investigation would take place and the culprits would be arrested and prosecuted. No police force would be disbanded because of this absolutely fictional scenario.
Factual error: The cap badge worn by Captain Halliday of the RAMC is the Queen's crown version, which didn't come in to use until the 1950's. It is also made of brass, when it should be in a bronzed finish for officers.