Not sure if bug or Hamachi problem, but the joined partner's game would some desync and have all enemies freeze. I as the host could still see him running and dooting, but enemies would deal no damage to him and he could not see anything else happening.
As a side note though, we loooove the game. Spent a bit of time streaming it and I could send you clips, but the issue wasn't visibly apparent from my point of view.