Although it is established early on that Selby is missing part of the third finger on his left hand, it can be clearly seen in many scenes that this finger is intact on both Selby and his doppelgänger.
During the demonstration of the ability to mold the skin on the face, the head wobbled like a stiff dummy.
When the plane lands at the airport in Chicago, you can clearly see mountains on the horizon. Being located in the Great Plains, there are no mountains near Chicago.
The Selby imposter claims the plan will succeed "without a shot being fired" but he himself shot Selby twice when he assumed his identity.
After the switch, the fake William Selby calls Ted Pearson "Phil" -- his real name.