As the trolley being driven by the scab played by Nathan Lane approaches the union protesters, there are no trolley tracks between the mob and the trolley. In the next shot, they suddenly appear in the dirt under young Francis' feet.
The story takes place in 1938. The diner used in the movie, Miss Albany Diner, is a Silk City model manufactured in 1941.
When Francis rides with the rag man, one of the telephone poles has a 1970s era transformer.
When the vigilantes' cars approach the hobo camp, they are observed across the steel-ribbed top of a boxcar with no running board, impossible in 1938.
As noted, this film takes place in 1938; but a billboard is seen advertising the Marx Brothers film "At the Circus" which is a 1939 release.
When Francis returns to his wife's house, the kitchen floor has 12-inch vinyl composite tiles (VCT tiles). While VCT did exist in the 1930s, they were only in the 9 inch size. 12 inch VCTs were not made until the 1970s.