Gold Bug: Ghost Fleets.

[Baron Munchausen]

I have seen something related to this, but it only happens to the AI. I've never had it happen to me in my own empire. Sometimes the AI will get two fleets 'confused', apparently by re-using a fleet name. That's what I guessed was the cause, anyway. This creates some sort of log-jam that freezes the ships involved and they sit there. Forever. I've sometimes taken control of an AI empire to remove the fleets and fix the problem (one or the other of the fleets can be accessed and emptied, which breaks the log-jam) but I've never figured out how to catch when it happens and send in a save game since it only happens to the AI.