The very first Nazi has his Kar98 leaning against a tree. Usually he picks up the rifle and shoots at you. However, you can also kill him before he picks up the rifle and it will still lean against the tree. If, however, you charge past him and swap your weapon for the Kar98 against the tree before he reaches for it, he will grab the weapon you left behind, but bring a Kar98 to bear on you. Your original weapon will still be leaning against the tree. Kill the Nazi and the gun he has in his hand disappears.