Recently I got an error on a sharded cluster when doing this:

sh.moveChunk("mip.sessions", { foo: "bar" }, "shard-02") 

ERROR: Collection is undergoing changes and chunks cannot be moved

The error message is rather misleading, I almost generated a ticket, because my application was idle and balancer was not running, so I did not understand where "undergoing changes" are coming from.

After some time, I found the reason, previously I run

db.adminCommand({ 
   setAllowMigrations: "mip.sessions", 
   allowMigrations: false 
}) 

Which blocked the sh.moveChunk() command.

 
A better error messages would be one of the following:

• Migrations are not allowed and chunks cannot be moved
• `allowMigrations` is set to `false` and chunks cannot be moved
• Collection is undergoing changes or migrations are not allowed and chunks cannot be moved