Earlier while in the middle of a typical JK run, my group noticed that we were missing a player. The player was still listed as being in the party, but he was no longer in the level. And by that, I don't mean he was AFK. He was gone completely. The player was with us in the last level and went down on the elevator with us. I don't recall if his character was on the elevator at the time of arrival at the level.

We would have just kicked him out, but the missing player happened to be the party leader (as it always seems to be when these things happen - Murphy's Law or something more telling?). Naturally, we could no longer progress in the level and had to leave. Fortunately it was only two levels after the sub-town, so the Energy lost was minimal.

I don't know how to replicate this issue, unfortunately.

Here's a screenshot I took before returning to haven, showing the non-existent person: Link.

Edit: SS was a bit small, here's a larger version: Link.