This issue only affects 5.3:
For a repl read-write, entity cache, if the failure happens on the async FutureUpdate call, that's fine because the Tombstone has already been sent and the cache won't return anything.
If the failure happens when the Tombstone is sent, we seem to have a problem because it results in stale data in the node that failed to apply the Tombstone. The FutureUpdate that comes after the Tombstone cannot apply because it doesn't find the Tombstone.
In 5.3, Sync logs any errors but does not propagate it, so it ends up with inconsistent state.