[Marshall V. Daut]

"...but wear on gear faces, shaft splines, and shifter fork slots can allow enough room for gear movement."

rotorwrench brings up a good point. While driving long distances in 2nd gear in "olden days", even at slower speeds, it was bad habit to rest one's right hand on the gear shift knob while the left hand stayed on the steering wheel. The pressure exerted on the long shift lever pushed the shift forks against the gear grooves and caused wear on the forks and the grooves' inner portions. That in turn also led to more slop and wear on the gear shifter ball at the bottom. Depending upon how much this happened over the years will explain why some gear shift levers flop around more than others. Wear in those three critical areas will definitely cause shifter slop. And that slop will also create a tendency for 2nd gear to move and then jump out of gear.
Marshall