I had a 5 man ZiS AT that was destroyed leaving a single crew member behind. I moved the guy back to try and reinforce and could, to a cap of 3. Then I proceeded to try and re-crew one of my own crew-less mortars with the now useless AT team and the game locked up. The map was Moscow and I was facing Wehrmacht.
Also the gunless crewmembers could not enter buildings of any kind.