Yes this does happen. I had it happen to a 60L 5spd. I think from memory the problem was there was too much lash or float on one of the gears. We had to put a wee packing shim under one of the gears. IIRC the endfloat allowed the gear to cock over slightly and jamb so it couldn't disengage. The packing shim stopped it from being able to do that and it was fixed. Strip you box down and see if the selector hub for 1st gear shows signs of having movement here.