We initially couldn't determine what was wrong. But, with the gearset out, we noticed the shift selector forks behaving poorly. Turns out that when Hercules made that last shift to third, he managed to send the shifter fork too far along its allowed travel and the little pin that tells the rest of the works, "Hey! I'm in third!" fell out. Here's a shot of the selector forks, the center one is tilted to show the hole that the little pin rides in.
And here's a shot of our troublesome little friend the pin.
With the little pin returned to its home, we put our transmission back together again.
We were very careful to make sure we got the shifting linkage reconnected. It would be a shame to have to take this thing apart again...
All sealed up, we threw the transmission back in the car in about 20 minutes.
Now to test it to confirm our repair worked!