Freeciv

At least the turn 307 one is an allied diplomat that gets killed.
We have never seen it as it's stealthy unit, but when server checks if client should have been aware of the unit, can_player_see_unit_at() has this code:

/* Allied or non-hiding units are always seen. */
  if (pplayers_allied(unit_owner(punit), pplayer)
     || !is_hiding_unit(punit)) {
    return TRUE;
  }

I think the part about allied units always being visible is simply wrong. That's not how other parts of the code are implemented. Shared vision would make more sense here.

Server tries to send unit several times (as packet_unit_short_info). I've debugged that all the way to the packets_gen.c code. But client side never receives it. This starts to look more like low-level networking code bug than server logic bug.

Marko Lindqvist wrote:

But client side never receives it.

That was user error. Client does know about the unit until it dies. Then it gets removed twice, and latter of those gives the error.

server_remove_unit_full() reduces vision before removing unit from onlookers for the very reason to avoid both of those actions potentially sending remove_unit. That doesn't work perfectly for stealth unit that loses V_INVIS vision but retains V_MAIN vision in vision reduction. Since it goes invisible with V_INVIS reduction, it gets removed. However, later removal of unit (because it has died) assumes it to be visible as can_player_see_unit_at() considers allied stealth units to be visible on seen (V_MAIN) tiles.