I had a mongodb with 2 shards, with a huge collection (14B rows).
I added another shard, and i'm getting the following errors when running sh.status()

active mongoses:
"3.2.9" : 1
balancer:
Currently enabled: yes
Currently running: yes
Balancer lock taken at Mon Oct 31 2016 07:08:03 GMT+0000 (UTC) by mshard1.XXXX:32232:1475250202:-218043174:Balancer:1442401236
Collections with active migrations:
triggerhood.newcube8 started at Mon Oct 31 2016 07:08:04 GMT+0000 (UTC)
Failed balancer rounds in last 5 attempts: 0
Migration Results for the last 24 hours:
989 : Success
1 : Failed with error 'aborted', from s1 to s3
1 : Failed with error 'aborted', from s1 to s2
476 : Failed with error 'chunk too big to move', from s2 to s3
3682 : Failed with error 'ns not found, should be impossible', from s1 to s3
504 : Failed with error 'chunk too big to move', from s1 to s3