Consider the following:

import scala.reflect._
class Param[T](val x: String) extends AnyVal
val x = classTag[Param[_]].newArray(10)
x(0) = new Param[Int]("foo")
// java.lang.ClassCastException: Param cannot be cast to java.lang.String

This shouldn't throw a ClassCastException. The issue is caused by the fact that classTag[Param\[_]] gives a ClassTag of String.

Strangely, the test run/valueclasses-classtag-existential does specifically check for this behavior.

Note that classTag[Param[X]] where X is some class, gives a ClassTag of Param (tested in run/valueclasses-classtag-generic). So this happens only with existential types.