Continuity mistake: When the German NCO begins to walk up the sand dune he has a stick grenade in his right boot. Next shot half way up the dune, the grenade has changed boots.
Continuity mistake: At the concentration camp, Mark Hamill opens an incinerator door, several doors along the row, and repeatedly shoots the German hiding inside. As we are looking out from inside the incinerator Lee Marvin appears behind him and says "I think you got him". The shot changes to a wide view outside but they are now looking in a different door - one of the first ones in the row.
Factual error: During all the major battle scenes involving tanks the Germans are using post-WWII tanks. The turret looks a bit like the M10 Wolverine with a top cover.
Factual error: At the very start of the film, as the Sergeant stabs a German soldier in the Great War, you get a good view of his "modern" rubber soled boots.