What causes compaction to retry?

[Wayne Conrad]

I've got a largish database that is being both compacted and 
synchronized concurrently.

$ curl --silent topaz:5984/_active_tasks | prettify_json.rb
[
  {
    "pid": "<0.313.0>",
    "type": "Replication",
    "task": "`874dbdaf26454065f33007eb42c8320a+create_target`: 
`http://sodium:5984/incoming/` -> `incoming`",
    "status": "Processed source seq 477428"
  },
  {
    "pid": "<0.24865.58>",
    "type": "Database Compaction",
    "task": "incoming retry",
    "status": "Copied 106794 of 159518 changes (66%)"
  }
]
At first the task was just "incoming", but when it got to 100%, it 
started over again with the task now being "incoming retry."  It's 
gotten to the end a few times now, each time starting over as a "retry" 
task.  What causes that?

Version: 1.2.0ac052866-git