Steps to reproduce:

1. Start up a stand-alone server, create a capped collection, create a unique index on _id on said collection.

   test> db.createCollection("foo",{capped:true,size:1000})
   { "ok" : 1 }
   test> db.foo.ensureIndex({_id:1},{unique:true})
   

2. Restart server as the first member of a replica set
3. Add another member to the replica set
4. The new member will have the collection foo, but no _id index.