We'll create 300 documents with a running index i (0..299). We will do a findAndModify 300 times at once, querying for i != 0 and updating i = 0 We'll see that a huge amount of findAndModify s fail. Steps: Run a MongoDB instance (3.2.16) on localhost:27017 Install the latest sbt Type sbt in the shell and the interactive sbt will start. sbt:mongobug> sbt:mongobug> set libraryDependencies += "org.reactivemongo" %% "reactivemongo" % "0.12.6" sbt:mongobug> set libraryDependencies += "org.slf4j" % "slf4j-api" % "1.7.25" sbt:mongobug> console scala> scala> :paste // Entering paste mode (ctrl-D to finish) import com.typesafe.config.ConfigFactory import reactivemongo.api._ import reactivemongo.api.collections.bson.BSONCollection import reactivemongo.bson._ import scala.concurrent.Await import scala.concurrent.ExecutionContext.Implicits._ import scala.concurrent.duration.Duration val driver = new MongoDriver(Some(ConfigFactory.empty()), None) val conn = driver.connection(Seq("localhost:27017")) val db = Await.result(conn.database("test"), Duration.Inf) val col = db.collection[BSONCollection]("col") Await.ready(col.remove(document()), Duration.Inf) for (i <- 1 to 300) Await.ready(col.insert(document("i" -> i)), Duration.Inf) val results = for (_ <- 1 to 300) yield { col.findAndUpdate( document("i" -> document("$ne" -> 0)), document("$set" -> document("i" -> 0)), sort = Some(document("i" -> 1)) ) } results.map(_.map(println)) // Exiting paste mode, now interpreting. After the application is ran, the output will look something like this: FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) FindAndModifyResult(Some(UpdateLastError(true,None,1,None)),Some(BSONDocument(<non-empty>))) FindAndModifyResult(Some(UpdateLastError(true,None,1,None)),Some(BSONDocument(<non-empty>))) FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) ... ... Every line that looks like this: FindAndModifyResult(Some(UpdateLastError(false,None,0,None)),None) means a failure in updating. When we remove the sorting, all the updates will succeed.