Revealing mistake: The Thing takes a swing at the captain after the captain opens the door. As the Thing swings his arm, he takes out a chunk of wood out of the door. If you step through the frames on the DVD, you can see the wood leaving the door a full second after the Thing has hit it. A small explosive charge that was not in sync with the actor. You can also see the outline of where the wood comes off before he swings his arm. And one more thing, his arm does not touch the door or even comes close to where the wood comes off - a safety precaution I presume.
Other mistake: When the captain comes face-to-face with the Thing for the first time, the upper right frame becomes blurred once the door starts opening until it closes followed by a scene change. Very odd except maybe they were trying to hide the minicharges to create the bullet holes after they close the door. The implanted charges are very visible when the scene is not blurred. Also there is a small charge in the wooden frame of the door near the door handle.