I defeated both sets of the Ninja Pigs from the quest Go Ninja, Go Ninja, Go! and was told by the rat Staff to continue looking for the Shedder without the Pigs' help. Running back to the main body of The Drains, I was pulled into a fight with ... no one. It was the area for the first Ninja Pig battle, but of course, I had already defeated them, so no one showed up!
I was not frozen; animation still worked properly, showing me in the idle battle position. But I could not move out of the spot, and the portal buttons were disabled presumably because I was in a "fight" and should use the Flee option. I had to quit and go back to the character selection screen, and when I rechose the same character, the game thankfully put me just outside of the Ninja Tunnel.
I took a screenshot, but the file is too big to be placed here
