Using Scala 2.12, the following expression throws an UndefinedBehaviorError over a ClassCastException:

new mutable.WrappedArrayBuilder(classTag[Unit]).result()

org.scalajs.linker.runtime.UndefinedBehaviorError: java.lang.ClassCastException: [V@1 is not an instance of Ljava.lang.Object;
    at $throwClassCastException (main.js:52:9)
    at $throwArrayCastException (main.js:58:3)
    at $asArrayOf_O (main.js:811:5)
    at $c_scm_WrappedArray$.make__O__scm_WrappedArray (WrappedArray.scala:105:10)
    at $p_scm_WrappedArrayBuilder__mkArray__I__scm_WrappedArray (ClassTag.scala:54:33)
    at $c_scm_WrappedArrayBuilder.result__scm_WrappedArray (WrappedArrayBuilder.scala:110:17)
    at $c_Lhelloworld_HelloWorld$.main__AT__V (HelloWorld.scala:17:62)
    at $s_Lhelloworld_HelloWorld__main__AT__V (HelloWorld.scala:14:8)

As the error message shows, we managed to create a [V, i.e., a void[]. This is not even supposed to be possible. It should be a scala.runtime.BoxedUnit[], which in Scala.js is actually a java.lang.Void[].

This regression appeared upstream in scala/scala#9091. That PR changed WrappedArrayBuilder to use ClassTag.emptyWrappedArray, which was defined as

  @transient private[scala] lazy val emptyArray: Array[T] = {
    val componentType =
      if (runtimeClass eq classOf[Void]) classOf[BoxedUnit] else runtimeClass
    java.lang.reflect.Array.newInstance(componentType, 0).asInstanceOf[Array[T]]
  }
  @transient private[scala] lazy val emptyWrappedArray: mutable.WrappedArray[T] =
    mutable.WrappedArray.make[T](emptyArray)

We copied that implementation in our overrides for ClassTag in
636be50
Unfortunately, it turned out that it was buggy. It was later fixed upstream in
scala/scala@5e6676d
but we didn't notice that, so we didn't port the fix.

This blew up when scala-collection-compat tried to upgrade to Scala.js 1.6.0 in scala/scala-collection-compat#462