Index: source/network/NetServer.cpp
===================================================================
--- source/network/NetServer.cpp
+++ source/network/NetServer.cpp
@@ -747,7 +747,7 @@
 	RemovePlayer(session->GetGUID());
 
 	if (m_ServerTurnManager && session->GetCurrState() != NSS_JOIN_SYNCING)
-		m_ServerTurnManager->UninitialiseClient(session->GetHostID()); // TODO: only for non-observers
+		m_ServerTurnManager->UninitialiseClient(session->GetHostID());
 
 	// TODO: ought to switch the player controlled by that client
 	// back to AI control, or something?
@@ -1402,7 +1402,7 @@
 	}
 
 	// Tell the turn manager to expect commands from this new client
-	server.m_ServerTurnManager->InitialiseClient(session->GetHostID(), readyTurn);
+	server.m_ServerTurnManager->InitialiseClient(session->GetHostID(), readyTurn, server.m_PlayerAssignments[session->GetGUID()].m_PlayerID == -1);
 
 	// Tell the client that everything has finished loading and it should start now
 	CLoadedGameMessage loaded;
@@ -1530,7 +1530,7 @@
 	m_ServerTurnManager = new CNetServerTurnManager(*this);
 
 	for (CNetServerSession* session : m_Sessions)
-		m_ServerTurnManager->InitialiseClient(session->GetHostID(), 0); // TODO: only for non-observers
+		m_ServerTurnManager->InitialiseClient(session->GetHostID(), 0, m_PlayerAssignments[session->GetGUID()].m_PlayerID == -1);
 
 	m_State = SERVER_STATE_LOADING;
 
Index: source/network/NetServerTurnManager.h
===================================================================
--- source/network/NetServerTurnManager.h
+++ source/network/NetServerTurnManager.h
@@ -1,4 +1,4 @@
-/* Copyright (C) 2018 Wildfire Games.
+/* Copyright (C) 2021 Wildfire Games.
  * This file is part of 0 A.D.
  *
  * 0 A.D. is free software: you can redistribute it and/or modify
@@ -18,16 +18,18 @@
 #ifndef INCLUDED_NETSERVERTURNMANAGER
 #define INCLUDED_NETSERVERTURNMANAGER
 
-#include <map>
 #include "ps/CStr.h"
 
+#include <map>
+#include <unordered_map>
+
 class CNetServerWorker;
 class CNetServerSession;
 
 /**
  * The server-side counterpart to CNetClientTurnManager.
  * Records the turn state of each client, and sends turn advancement messages
- * when all clients are ready.
+ * when all non-observer clients are ready.
  *
  * Thread-safety:
  * - This is constructed and used by CNetServerWorker in the network server thread.
@@ -43,13 +45,13 @@
 	void NotifyFinishedClientUpdate(CNetServerSession& session, u32 turn, const CStr& hash);
 
 	/**
-	 * Inform the turn manager of a new client who will be sending commands.
+	 * Inform the turn manager of a new client
+	 * @param observer - whether this client is an observer (will be sending commands, must account for lag).
 	 */
-	void InitialiseClient(int client, u32 turn);
+	void InitialiseClient(int client, u32 turn, bool observer);
 
 	/**
-	 * Inform the turn manager that a previously-initialised client has left the game
-	 * and will no longer be sending commands.
+	 * Inform the turn manager that a previously-initialised client has left the game.
 	 */
 	void UninitialiseClient(int client);
 
@@ -73,6 +75,9 @@
 	/// The latest turn for which we have received all commands from all clients
 	u32 m_ReadyTurn;
 
+	// Client ID -> whether they are only an observer.
+	std::unordered_map<int, bool> m_ClientsObserver;
+
 	// Client ID -> ready turn number (the latest turn for which all commands have been received from that client)
 	std::map<int, u32> m_ClientsReady;
 
Index: source/network/NetServerTurnManager.cpp
===================================================================
--- source/network/NetServerTurnManager.cpp
+++ source/network/NetServerTurnManager.cpp
@@ -76,6 +76,9 @@
 	// See if all clients (including self) are ready for a new turn
 	for (const std::pair<const int, u32>& clientReady : m_ClientsReady)
 	{
+		// Ignore observers.
+		if (m_ClientsObserver[clientReady.first])
+			continue;
 		NETSERVERTURN_LOG("  %d: %d <=? %d\n", clientReady.first, clientReady.second, m_ReadyTurn);
 		if (clientReady.second <= m_ReadyTurn)
 			return; // wasn't ready for m_ReadyTurn+1
@@ -145,7 +148,8 @@
 			if (hashPair.second != expected)
 			{
 				// Oh no, out of sync
-				m_HasSyncError = true;
+				if (!m_ClientsObserver[clientStateHash.first])
+					m_HasSyncError = true;
 				OOSPlayerNames.push_back(m_ClientPlayernames[hashPair.first]);
 			}
 		}
@@ -171,13 +175,14 @@
 	m_ClientStateHashes.erase(m_ClientStateHashes.begin(), m_ClientStateHashes.lower_bound(newest+1));
 }
 
-void CNetServerTurnManager::InitialiseClient(int client, u32 turn)
+void CNetServerTurnManager::InitialiseClient(int client, u32 turn, bool observer)
 {
 	NETSERVERTURN_LOG("InitialiseClient(client=%d, turn=%d)\n", client, turn);
 
 	ENSURE(m_ClientsReady.find(client) == m_ClientsReady.end());
 	m_ClientsReady[client] = turn + COMMAND_DELAY_MP - 1;
 	m_ClientsSimulated[client] = turn;
+	m_ClientsObserver[client] = observer;
 }
 
 void CNetServerTurnManager::UninitialiseClient(int client)
@@ -187,6 +192,7 @@
 	ENSURE(m_ClientsReady.find(client) != m_ClientsReady.end());
 	m_ClientsReady.erase(client);
 	m_ClientsSimulated.erase(client);
+	m_ClientsObserver.erase(client);
 
 	// Check whether we're ready for the next turn now that we're not
 	// waiting for this client any more