Desync
Also said as Out of sync, Determinism bug, Divergence
Machines running the same game stop agreeing about it. In a game that sends only inputs, one tiny difference grows until the players are in two different matches.
It is among the hardest bugs in the trade: it can come from a rounding difference on one model of processor, and nothing looks wrong until minutes later. Games detect it by comparing checksums of the state.
Say it
“Compare a checksum of the whole game state every 30 ticks. If they differ, save both states to a report and stop the match. Do not use floats or unsorted containers in the simulation.”
Where you’ve seen it
Factorio simulates the whole factory deterministically on every client from the actions received, and in the developers' words, "if it's ever different from the server or any other client, a desync occurs".
Not to be confused with lag
Lag means you are seeing the truth late. A desync means there is no longer one truth. Waiting fixes lag, and it never fixes a desync.