If memory serves me, there was a large fire in the records center in St Louis back in the early 70's. Most of the records lost were from WW2.
Actually, I think most of the lost records were Korean era? I know my dad's records were lost and he served in the Air Force from '50 to '55.
Edit: Here's what I found at Wikipedia...
Affected records
The losses to Federal military records collection included:
80% loss to records of U.S. Army personnel discharged November 1, 1912, to January 1, 1960[2]
75% loss to records of U.S. Air Force personnel discharged September 25, 1947, to January 1, 1964, with names alphabetically after Hubbard, James E.[2]
Some U.S. Army Reserve personnel who performed their initial active duty for training in the late 1950s but who received final discharge as late as 1964.
None of the records that were destroyed in the fire had duplicate copies made, nor had they been copied to microfilm. No index of these records was made prior to the fire, and millions of records were on loan to the Veterans Administration at the time of the fire. This made it difficult to precisely determine which records were lost.