In implementing NODE-1864, we discovered that unordered bulk writes had a chance of failing if the following conditions were met:

• retryWrites=true
• # of retryable batches > 1

This has to do with the fact that UnorderedBulkOp currently executes all of its operations in parallel while using the same session (which necessitates serial operations).

The solution is to make UnorderedBulkOps execute each batch in parallel.