Well, you got me there. The allen wrench is not very snug in the hole. Would that cause the shifter levers to bind if they are just a little bit out of adjustment?
ABSOLUTELY it would.
The shift lever rods that go through to the inside of the case have lockouts built into them so that there is no possible way for the trans to end up in two gears at once.
If one is being held a little off it's center, the others will not move.
I'd be willing to bet something on your linkage moved and it just needs readjusted.