I had a '67 Cougar and it seemed like it was going from gear to gear VERY easily, at about 120,000 miles. I took the console apart, and used a strong metal rasp on all the detents, to recreate them as near to new as possible.
They really looked worn down, so it was easy to see how they could slip from N to D, for example....

Those Fords from the mid 70s were pretty bad. I used to drive a 1978 LTD II, it had those transmissions that could slip into gear, it was a slug that had a healthy appetite for Persian Perrier, and if you hit the brakes hard it would nosedive, lock up and the rear end would break loose. Very scary.
Some company car that turned out to be......
Just about every one of those hit the scrap yards years ago.

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1991 560 SEC AMG, 199k <---- 300 hp 10:1 ECE euro HV ...

1995 E 420, 170k "The Red Plum" (sold)

2015 BMW 535i xdrive awd Stage 1 DINAN, 6k, <----364 hp

1967 Mercury Cougar, 49k

2013 Jaguar XF, 20k <----340 hp Supercharged, All Wheel Drive (sold)