When Nick steals a motorcycle you can see that it is an old opposed twin BMW. But for all the stunt riding it is clear to see that he is not on the same motorcycle, he is using a single cylinder dirt bike.
During one of the group's shootouts against the Nazis, Pvt. Fred Canfield is shown smoking a brand new cigar while hiding in a dirt hole. In the next shot, the cigar is much smaller.
During several of the battles, the same actors can be seen dying multiple times, only from different camera angles.
When Bo Svenson and Ian Bannen first enter the German train and pass four guards, the guard on the far left breaks character and starts to laugh at the end of the shot.
A wire can be seen pulling a soldier during a grenade explosion in the first battle with the Waffen-SS.
When Nick played by Michael Pergolani is updating the paper work he uses 'Liquid Paper/Correction Fluid' to alter the document. Liquid Paper was not invented until 1951, and was only really widely available in Europe from the mid 70's onwards.