Near the beginning of the movie, Fred Derry says he met his wife Marie when he was in training in Texas. Later on, Fred tells Marie that he'll fix her a nice meal, like he used to do at the drugstore. Fred was no longer working at the drugstore when they met.
After Fred Derry dresses after staying the night at the Stephensons' home, he emerges in his uniform with his jacket belt-buckle positioned way off to the right, over his right front pant's pocket. Any real serviceman would position his belt buckle (pants or jacket) dead-center, directly over his pant's zipper flap.
When the three war veterans board the B-17, the insignia on the side of the fuselage is correct for post-World War II (after September 1945), a white star on a blue disc with white bars on the left and right sides of the disc. However, when the B-17 is taking off, there is a brief glimpse of just a white star on a blue disc with no white side bars, the official insignia used from August 1942 to June 1943.
When Fred and Al are talking in the nose of the B-17 while Homer is sleeping, the hair on Al's forehead changes twice during the same conversation--from a curl to a general muss and then back to the same curl again.
During Al Stephenson's speech at the bank get-together party, he talks about his division being in Okinawa. The 25th Infantry Division (Tropic Lightning), was part of the Luzon campaign that started January 1945 and ended with the divisions relief at the end of June 1945. At no time was the 25th on Okinawa. The divisions that did participate in the Okinawan campaign (April 1 - June 22, 1945) were the: 7th, 96th, 27th, and 77th Infantry Divisions of the army and the 1st, 2nd, and 6th Marine Divisions.
When Fred Derry climbs into the B-17's nose compartment at the salvage field, the nose section noticeably wobbles as he steps into it, indicating the nose is a separate, detached set piece. The nose section on a complete, intact B-17 would not move like that.