A desync means one player’s game no longer matches the server’s. The game cuts that player off and offers them a way back in. The server keeps running. A bug in a mod is the most common cause. If it keeps happening, test with the newest mods turned off.
What it looks like
The player is disconnected, waits while the game collects a report, then is offered a way to reconnect. On the panel, the Crashes page shows A player desynced on that start. It says one player’s game no longer matched the server’s and that they were sent back to rejoin.
A desync is a disagreement about the game itself, not a slow connection.
The player rejoins
Most of the time, reconnecting works. If the player is dropped again at once, restart the server and have them join again. A restart drops everyone who is on, so warn them first with Message everyone on the Players page. A restart does not remove the cause.
Find the mod
When mods are on, a desync that comes back is usually a mod, or an update to one. Work from the newest change.
- Open the Mods page and look at Installed. Note which mods you added or updated lately.
- Turn off the newest one with the switch on its row (Turn (mod) off). If you updated a mod, you can turn it off and compare.
- Start the server. Mod changes load at the next start.
- Play as before. If the desync stops, that mod was the cause.
- Turn it on again and repeat the action that caused the desync to confirm it.
See save a copy.
Tell the mod’s author which mod it was and what the player was doing when it happened. A desync report does not name the mod for you, so your own test is the evidence.
If no mod is on
If the server runs no mods besides the game’s own, the cause is a bug in the game itself. Report it to Wube on the Factorio forums. Include what the player was doing.
If it does not work
- One player desyncs and the others do not. The player’s game must match the server’s version and mods exactly. See mods mismatch on join.
- The server stopped too. That is a crash, not a desync: see a mod crashed the server.
- You want the exact moment. Read the server logs.
More about the game: Factorio servers.
Sources: wiki.factorio.com: Desynchronization, wiki.factorio.com: Multiplayer