We find quite a number of these errors in our cr-syncer logs.

2021/01/26 16:40:16 Syncing key "default/1710.2053438" from queue "downstream" failed: WarehouseOrder default/1710.2053438 @ 184236994: update status failed: Operation cannot be fulfilled on warehouseorders.ewm.sap.com "1710.2053438": the object has been modified; please apply your changes to the latest version and try again

cr-syncer does not retry applying the change as the error message suggests. If there are other update events from downstream cluster, it will get in sync again. But if there are no more change events from downstream for that CR it remains in a not synced state.
When a CR is in this state it even does sync changes from upstream cluster anymore.
It remains in this inconsistent state until cr-syncer is restarted.