Happened to me this morning on the 1965, happens a lot on the 1968, I drove to the pump, and just before I entered my car didn't get out of it's 3rd gear, the shifter got somewhere stuck in between.

I eventually pulled it back with the clutch pressed and pulled the rod next to the steering column back so it got back into gear again.
(not my pic)
http://image.popularhotrodding.com/f/10495315+w750+st0/0811phr_15_z+1968_chevy_chevelle_cpp_saginaw_600_s teering_box+u_joint_in_place.jpg


Have I just been careless or is it time for maintenance due to part wearing?

thx in advance. :)

You have the normal wear. Look for screws near the end of the column jacket that are located in some angled slots. Loosen the screws and rotate the piece in the end of the column so that it applies pressure to the shift levers. That should take up the slack that allows the levers to slip out of position.