If those springs or lockout arms aren't working correctly I could see that you may be able to shift into the other gears while second was still engaged.
The mechanism in the side cover is meant to never be able to engage 2 gears at once. If it's damaged, or a spring came off you can engage 2 gears at once locking the output shaft.
Shift forks can break also, never have seen it in a Muncie, but have seen it in truck transmissions.
Probably worth a try to remove the side cover in car to fix the problem without pulling the transmission. Even if it's not in the side cover little time lost to save a transmission R&R.
__________________
Brad Yost
1973 T/A (SOLD)
2005 GTO
1984 Grand Prix
100% Pontiacs in my driveway!!! What's in your driveway?
If you don't take some of the RACETRACK home with you, Ya got cheated 
  
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