Mongo Shard with Negative Average Size? I'm seeing the following behavior from a shard in mongo:

mongos&gt; db.fs.chunks.getShardDistribution()

Shard SH0
 data : 740.29GiB docs : 5003098 chunks : 32949
 estimated data per chunk : 23MiB
 estimated docs per chunk : 151

Shard SH1
 data : 800.67GiB docs : 5413760 chunks : 32948
 estimated data per chunk : 24.88MiB
 estimated docs per chunk : 164

Shard SH2
 data : -188212B docs : 3083 chunks : 37
 estimated data per chunk : -5087B
 estimated docs per chunk : 83

Totals
 data : 1540.96GiB docs : 10419941 chunks : 65934
 Shard SH0 contains 48.04% data, 48.01% docs in cluster, avg obj size on shard : 155KiB
 Shard SH1 contains 51.95% data, 51.95% docs in cluster, avg obj size on shard : 155KiB
 Shard SH2 contains -0.01% data, 0.02% docs in cluster, avg obj size on shard : -115628438B


I'm attempting to drain SH2, but 37 "jumbo" chunks are left. This is a gridfs collections sharded on files_id, n - it's hard to believe these chunks are actually jumbo, but I haven't been able to do anything to split them (I've tried everything mentioned in the other relevant SO questions about splitting jumbo chunks).

Any ideas, any clue why this is showing up like this?

EDIT

sh.status() outputs: 
mongos&gt; sh.status()
--- Sharding Status ---
  sharding version: {
    "_id" : 1,
    "minCompatibleVersion" : 5,
    "currentVersion" : 6,
    "clusterId" : ObjectId("546bf1a7c0671b92b546fb90")
}
  shards:
    {  "_id" : "sh0",  "host" : [redacted] }
    {  "_id" : "sh1",  "host" : [redacted] }
    {  "_id" : "sh2",  "host" : [redacted],  "draining" : true }
  balancer:
    Currently enabled:  yes
    Currently running:  no
    Failed balancer rounds in last 5 attempts:  0
    Migration Results for the last 24 hours:
        8 : Success
        1 : Failed with error 'migration already in progress', from syn-sh_1 to syn-sh_0
  databases:
   .....
    {  "_id" : "Files",  "partitioned" : true,  "primary" : "sh0" }
        Files.fs.chunks
            shard key: { "files_id" : 1, "n" : 1 }
            chunks:
                syn-sh_0    32967
                syn-sh_1    32968
                syn-sh_4    37
            too many chunks to print, use verbose if you want to force print


EDIT 2
When attempting to move, Mongo thinks the chunk size is approx 1200 Petabytes

{
    "cause" : {
        "chunkTooBig" : true,
        "estimatedChunkSize" : NumberLong("1284608926708044072"),
        "ok" : 0,
        "errmsg" : "chunk too big to move",
        "$gleStats" : {
            "lastOpTime" : Timestamp(0, 0),
            "electionId" : ObjectId("553034c33e622e6efa8c0a6c")
        }
    },
    "ok" : 0,
    "errmsg" : "move failed"
}
Reboot of affected shards fixed it.

That's kinda mongo in a nutshell - I ended up submitting a bug and that was their engineer's advice, and it happened to work.