Visible crew/equipment: When Jason, his brothers and all his men are marching down the wet street, tire treadmarks from the camera truck are visible in the mud under their feet.
Deliberate mistake: Robert Brown wears a gold turtleneck sweater with his leather outfit throughout the series. Though it's more than 100 years beyond the show's 19th Century setting, he and the producers decided that he looked good in it, so allowed the anachronism to stand.