Replicaset stop replication

We get following error in our secondary, this is second time we encounter the problem, we have remove all the data in secondary and let it resync all the data in one week ago and seems fixed. But the problem comes again.

"errmsg" : "syncTail: 10068 invalid operator: $id, syncing: { ts: Timestamp 1342667002000|51, h: -3142433914917806080, op: \"u\", ns: \"boc.paper\", o2: { _id:

{ $id: \"4fe0050d217042a83c010000\" }

}, o: { $set:

{ info.difficulty: [ \"1\", \"2\", \"3\" ] }

} }"
},