Scala closures capture outer variables, which can lead to unhappy serialization when you don't expect a lazy data structure under the covers. The lesson is, for serialization, favor strict, concrete types.

StreamOuter.scala

class A
class B extends Serializable

val baos = new java.io.ByteArrayOutputStream(1024)
val oos = new java.io.ObjectOutputStream(baos)

val streamSurprise: Seq[A] = Stream.fill(3)(new A) // say you don't know it's a Stream under the covers
val transformation = streamSurprise map (a => new B)

oos.writeObject(transformation) // fails due to NotSerializableException: A

oos.writeObject(transformation.toList) // succeeds