Erm...WAT.

wat2.scala

class Thing {
  var x = 5
  override def equals(o: Any) = o match {
    case that: Thing => that.x == this.x
    case _ => false
  }
}
val a = new Thing
val b = new Thing
val stuff = scala.collection.immutable.Set(a, b)

println(stuff.size) // Prints 1.
println(a equals b) // Prints true

b.x = 10
println(stuff.size) // Still 1
println(a equals b) // Now false.
println(a.x, b.x)   // Prints (5,10)