Desynchronization
A desynchronization (desync) is when during multiplayer a player's game becomes desynchronised from the Factorio server. Their game has become different from the server's game.
What are desyncs
Desyncs can be caused either by a bug in the core Factorio game code or by a bug in a mod/scenario.
Factorio multiplayer architecture
Factorio multiplayer code uses Deterministic Lockstep to synchronize clients, this is a method of synchronizing a game from one computer to another by sending only the user inputs that control that game, rather than networking the state of the objects in the game itself. It means that all players need to simulate every single tick of the game identically. If any computer does something ever-so-slightly different, a desynchronization (desync) occurs. The game includes processes to ensure any issues with network traffic does not cause desyncs.
Player experience of desyncs
Upon a desync between a player and server occurring that player immediately disconnects from the server and starts to reconnect. This reconnection isn't to allow the player to play the game, but to obtain the server's copy of the save game (map and game state) to include in a desync report. After the reconnection is complete the player is offered the options to save the report, quit or reconnect to the server to play.
The desync report can be provided to either the mod/scenario makers or the Factorio game developers to help them resolve the bug. Depending upon the exact cause of the desync the player may be able to reconnect to the player and continue playing, or they may immediately desync on reconnecting.
Desync Report
The desync report contains both the player's and server's copy of the save game (map and game state). The player's copy is from when the desync occurred and titled desynced-level. With the server's copy being from when the automatic reconnection was made and titled reference-level.zip. A copy of the players Factorio log file is included titled log.txt.
The desync report does not state the cause but is a record of the game state at the time of the desync.
Desync investigations with mods/scenarios involved
The desync report is a useful reference for investigating desyncs, however, ideally, it is supplemented by an account of what was occurring and being done by players at the point of the desync occurring.
include:
https://lua-api.factorio.com/latest/Data-Lifecycle.html
https://wiki.factorio.com/Tutorial:Modding_tutorial/Gangsir#Multiplayer_and_desyncs
Desync reporting with only base game
If no mod or scenario was involved in the desync, the desync report and activity information should be posted to the Factorio forums bug report section.