Factual error: The "Blue Baron" tells the dancer that the Kaiser gave him a certain medal. In fact, the medal he points to (and the girl fondles) is a WW2 repeat badge to the Iron Cross first class, instituted in 1939 to denote presentations of the Iron Cross first class to personnel who had already received it in WW1. He may have received the original Iron Cross from the Kaiser, but by the time he had a chance to receive that repeat badge, the Kaiser was long through handing out medals. (00:15:10)
Continuity mistake: At the airfield, Carter and Newkirk go to move, Carter steps left, and falls into the water. Newkirk takes a step, and leaps into the water, losing his hat, and trying to make it look like Carter pulled him, but you can see him looking to the way he falls. When the camera angle changes, Carter is splashing about, and Newkirk walks up to him from the left without having gotten out from where he fell. And his hat is on, despite losing it from jumping in.
Revealing mistake: When Hogan, Newkirk, and Carter are at the Blue Baron's secret airfield, Hogan opens the barrel and stuffs in a rag and lights it so it will be a flare for the Allied bombers. The flight line and the planes on it are an obvious cutout. The fire couldn't have spread to the surrounding ground like it was shown. Also, when the bombs were going off on the airfield, you can see that the explosions are very small charges behind the cut outs.