Revealing mistake: When Landfill's wake is knocked over and his body falls on the guy, his eyes are open for a second or two before the scene ends.
Revealing mistake: When the American and German teams are playing a drinking game involving putting coins into glasses while downing pints, the camera switches to an angel at the end of the table. If you look at the front two people's hands, they are going past the pint glasses, and the coins are disappearing, yet the pints are still filling up with coins.
Revealing mistake: During the fight between Cherry and Landfill, you can see several times that it is not Cherry, but a stunt double. The stunt double is much thinner and has much lighter skin. It is best seen when she is on the ground and Landfill is kicking her in the stomach.
Answer: Overconfidence. The Germans are convinced that they'll never master Das Boot, so they're too busy watching for the Americans to fail to concentrate on their own boot.
Captain Defenestrator