Ragtop nailed it. The cheap ass nylon bushings on the auto shifter wear and the thing will drop into reverse. My cement mixer got crushed a bit between my brother's 69 Mustang and his fence. Of course the Mustang was bent a bit too, but it wasn't mine, dammit.
When I bought my POS 65 Fastback the shifter was so sloppy I could move the shifter 4 or 5" laterally.