Detective James McParland goes undercover among the Pennsylvania miners, claiming to be from Belfast. At no point does Limerick-born Richard Harris even attempt an Ulster accent.
Sean Connery takes out a $1 bill which wasn't printed until 1923.
The movie is supposed to take place fifty years earlier.
The steam locomotive has a steel riveted cab, clearly of 1890's or later vintage, rather than an 1870's design.
When Detective McParlan (James McKenna) and Mary Raines are walking down the Philadelphia street, a large hill can be seen in the background. There are no hills in the vicinity, or visible from, downtown Philadephia.