From Cryptosmith:

The NSA had an incinerator in their old Arlington Hall facility that was designed to reduce top secret crypto materials and such to ash. Someone discovered that it wasn’t in fact working. Contract disposal trucks had been disposing of this not-quite-sanitized rubish, and officers tracked down a huge pile in a field in Ft. Meyer.

How did they dispose of it? The answer is encrypted in the story’s text!

The story sounds like it’s from the early 1960s. The Arlington Hall incinerator contained a grating that was to keep the documents in the flames until reduced to ash. The grate failed, and “there was no telling how long the condition had persisted before discovery.”