Trouble identifying "Tombstone exists for queued item; deleting." in ipad logs
See: Tombstone (data store). 

In brief, a tombstone is a token in a distributed data storage system that gets put in place when a record is deleted. It's needed for synchronizing different stores on the distributed system. For example, if record 'A' is deleted on one store, when the system tries to sync it may see another copy of record 'A' on a different store, and it won't know that the record was deleted from the first. It might unwittingly sync record 'A' back to the first store, effectively undeleting it. The tombstone tells the system that record 'A' was deleted on the first store, so that the system can sync that deletion to the other stores appropriately.