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.
Visible crew/equipment: When the airplane flies over the area where the saucer skidded in and became buried in the ice, you can see the equipment used to clear the snow and smooth out the ice (at the narrow end of the ice). You can also see the tracks from the machine criss-crossing the circular portion from the aerial shots.