Assertion on BSON size (0xEEEEEEEE)

Shard server log repeatedly shows assertion on invalid object size. Looks like corruption:

Mon Feb 13 07:15:55 [rsSync] replSet syncing to: mongo1:27000
Mon Feb 13 07:15:56 [rsSync] Assertion: 10334:Invalid BSONObj size: -286331154 (0xEEEEEEEE) first element: EOO
Mon Feb 13 07:15:56 [rsSync] replSet syncTail: 10334 Invalid BSONObj size: -286331154 (0xEEEEEEEE) first element: EOO, syncing: { ts: Timestamp 1329088027000|19, h: -1120214476444430649, op: "i", ns: "anodelogs.log", o: { timestamp: new Date(1329088027621), level: "info", message: "Sending keep alive request to app "perfmonitor.saary" via http://10.28.148.177:30000 RW=yosefd2/__apps/saary/perfmonitor/.shimmed.v2.index.js", meta:

{ time: new Date(1329088027543), app: "ka.sys", domain: "anode-yosefd2.cloudapp.net", instanceId: "anodejsrole_IN_1", requestID: null, stackTop: "c:\an\repo\yosefd2\sys\ka\index.js:31:25", args: [ "Sending keep alive request to app "perfmonitor.saary" via http://10.28.148.177:30000", "RW=yosefd2/__apps/saary/perfmonitor/.shimmed.v2.index.js" ] }

, _id: ObjectId('4f38461bbf5d8f040700b119') } }

This repeats every 30 seconds and mentions the same object.