I walked into a cave, and the entrance disappeared. I started getting hit by invisible units (bats) immediately, and I was knocked out. I continued to see my HHP deplete by something I couldn't see, until I was logged out automatically. When I logged back on, there were 3 bats right at the entrance of the cave, the cave entrance had reappeared, and I had under 10 HHP left. (Started at about 90).
Close call, but I think it was caused by the bats spawning too close to the entrance of the cave.