While running a major batch run, which updates a lot of documents, we are occasionally seeing this error:

MongoDB.Driver.WriteConcernException: WriteConcern detected an error 'passes >= maxPasses in NamespaceDetails::cappedAlloc: ns: local.oplog.rs, len: 1060528, maxPasses: 35352, _maxDocsInCapped: 2147483647, nrecords: 8019946, datasize: 8426545700'. (Response was { "ok" : 1, "code" : 10345, "err" : "passes >= maxPasses in NamespaceDetails::cappedAlloc: ns: local.oplog.rs, len: 1060528, maxPasses: 35352, _maxDocsInCapped: 2147483647, nrecords: 8019946, datasize: 8426545700", "n" : NumberLong(0), "updatedExisting" : false }).

It causes an exception in the client (.NET) application.
Apparently the error is generated in cap.cpp
As far as I can tell from the error message, the document is 1060528, and it gives up after having removed 35352 documents from the oplog.
We have seen this error 8 times since July 5 (on a job that runs daily).
len varies between 200k and 1600k