There are Hollywood gaffes a-plenty, usually in the earlier films like John Wayne's "The Horse Soldiers"... 45-70 trapdoor carbines are the weapon of choice as well as Indian War leather gear and accoutrement/belt plates. They also used a mix Colt SSA and correct Civil War Colt and Remington revolvers. Movies from the 1950s and 1960s often used original gear from the Span-Am War and Indian Wars and sometimes even the Civil War, because the gear was still around and relatively plentiful.
In "Glory" - a movie where they really tried to do a good job with accuracy - there's a scene of Col Shaw blowing dust out of the works of his brass framed . 51 Navy Colt as they assualt Battery Wagner. Colt didn't make brass framed revolvers. My favorite "Glory" mistakes are the obvious use of rubber rifles during the assault on Battery Wagner - look at the assault scenes where artillery rounds cause men and weapons to fly through the air...the rifles bend in flight. Also, look for Shaw's branch of service (color of his shoulder boards) to change early in the movie while their on the march. The continuity supervisior must have been at lunch...
I had the opportunity to meet Jeff Daniels a couple years ago and we talked about his portrayal of Chamberlain in "Gettysburg". He said he really studied Chamberlain and wanted to show him as the thinker he was (college professor, college president and post war Gov of Maine) so it's no surprise that he actually changed the cylinder correctly in the movie. "Dumb & Dumber" aside, he really gets into his roles.... Gettysburg is pretty accurate because they depended on the reenactment community for most of the extras and gear and these reenactors were pretty well supervised and regulated for authenticity. Although there are always issues like the capture of the 300 lb Confederate - something you just didn't see in 1863...