Revealing mistake: When George is on the bridge and says "I wanna live again", and Burt drives up, in the shot where George hugs Burt, then Burt pats him on the back, you can see Burt's handprint in the fake snow that was used (a mixture of foamite soap and water) on the back of George's coat. (02:02:35)
Revealing mistake: When CG and IB are on the plane, watch the scenery in the window. It appears to be going in reverse.
Revealing mistake: When Ballin's plane flies and finally crashes, it becomes obvious that the horizon is a still image with the film of the explosion superimposed directly onto it.
Revealing mistake: When Fred Derry gets on the B-17 at the end of the movie to exercise his demons, the nose section of the plane noticeably moves when he gets on and gets off, showing it's a studio prop.