Deliberate mistake: There are no lights ever to be found in the underground world and the ceiling is literally the surface's ground, but there is still a perfect level of lighting regardless.
Deliberate mistake: The German tanks in this movie are not really Tiger tanks. They are Russian T-34's made to look like Tigers. The T-34's are done up very realistically, including Zimmerit (the wavy pattern on the hull of the tiger), which was an anti-magnetic paste put on the tank.