Branch delayed; Wrong predecessor - "very rare"
This just happened to us. Arthur says it should be very rare. Still an issue though. Appears as though the endorser constructed the operation for 521696 but the local node had not switched to the new head yet? Some type of race condition or synchronization issue between endorser and node?
Hoping there is a fix to make the endorser smarter when interacting with the node to make sure it is on the latest head block.