After upgrading yesterday from MongoDB 2.4.x to MongoDB 2.6.x we hit the exact same error.

In our case on an update() command.

collection.update({'_id': {'$in': remote_ids}}, {"$set": {"_oc": True}}, multi=True, w=1, j=True)

When the _oc flag is set entries are eventually removed. Although the update operation ended in an exception it seems that the flag was correctly set. Since the documents where removed and the error was not reported again for the document.

It's unclear if the document in question was updated correctly or was reprocessed without an error later for a second time.

Can MongoDB confirm or deny that this could happen ?

Running .validate(true) on the collections returned no errors.

Is there any more information about this issue from the MongoDB site ?
How could this corruption occur ?
Could this be related to upgrading from 2.4 to 2.6 ?

We have more MongoDB instances that we would like to upgrade but these kind of corruption issues make us very scared to do so.