scala> Future.sequence(List.iterate(Future(1), 9000000)(a => a))
java.lang.OutOfMemoryError: GC overhead limit exceeded
  at scala.collection.mutable.ListBuffer.$plus$eq(ListBuffer.scala:174)
  at scala.collection.mutable.ListBuffer.$plus$eq(ListBuffer.scala:45)
  at scala.collection.generic.GenTraversableFactory.iterate(GenTraversableFactory.scala:247)
  ... 29 elided