A panther G disguised as an American M10 tank destroyer used in Operation Greif during the battle of the bulge.
The overall profile is very convincing only the running gear betrays its identity. It is reported the muzzle break was removed during road marches and put back on when combat maybe near.
US Army intelligence was shocked and afraid that the Germans had turned to such measures. No one knows exactly how many were actually used but US army intelligence suggests there were around 10.