Class Enumeration defines method values which returns the values of an enumeration as a set.
This method computes this set lazily and is defined as follows:

/** The cache listing all values of this enumeration. */
@transient private var vset: ValueSet = null
@transient private var vsetDefined = false

def values: ValueSet = {
  if (!vsetDefined) {
    vset = new ValueSet(immutable.SortedSet.empty[Int] ++ (vmap.values map (_.id)))
    vsetDefined = true
  }
  vset
}

I claim, that the field vsetDefined has to be declared @volatile, otherwise a thread invoking values might see vsetDefined as true, but may not see the actual value in field vset, i.e. may see vset still to be null. If vsetDefined were declared @volatile, a happens-before relation would be established between writing field vset and reading it.