My brother bought a 1976 Zetor 6718 with 9800 hrs on it cheap, because it popped out of 5th. (he traded a ferguson TEF against it, converted with a Perkins P3 that smoked as much from the crankcase breather as from the exhaust)
It was caused by the fuel tank/dashboard rubbers that were sagging, causing it to hang against the dashboard mounted gearlever.
With the tank/dash rubbers replaced it still popped out. We opened the transmission and found that meanwhile, a 5ct Seeger retaining ring was missing of the 5th speed shifting collar. It took 4 evenings to fix it, it could have been done quicker if the previous owner used new rings remounting the other parts after trying to fix it himself: We found tack welded retaining rings in the transmission, as far as it was straightforward. (you could see that the retaining rings were damaged from screwdrivers, instead of proper pliers) When he got to the point that you need to pull out the PTO shaft from behind, to drop the PTO gears in front of the travel gears, to get access to the main gearbox, the guy gave up and put everything back together, tack welding the retaining rings instead of putting in 3 dollars of new rings
We restored the tack welding by the previous owner, by some more proper retaining rings, shimmed the play out of it, and this 70 hp tractor is doing everything from pulling 16 ton dump trailers to land levelling and wood chipping for a highway maintenance crew...
After a total rebuild of my 3011 transmission last winter, i can dream these transmissions and probably rebuild one in a day, not counting the time to pull the gearbox from the tractor.