This only seems to be a problem with old-style write operations (wire_version 0) and not with the new write commands.

We connect to a standalone mongod and use an empty collection.

bulk = collection.initialize_ordered_bulk_op
bulk.insert({:a => 1})
bulk.insert({:a => 2})
bulk.execute({:w => 2})

We expect this call to raise a BulkWriteError, and it does. However, if we examine the collection afterwards, it will contain the first document we tried to insert,

Running the same steps with an unordered bulk op:

bulk = collection.initialize_unordered_bulk_op
bulk.insert({:a => 1})
bulk.insert({:a => 2})
bulk.execute({:w => 2})

results in both

and

being inserted into the collection.