Example code:

val (x, y) = Iterator.iterate(0)(_ + 1).take(6).span(_ != 1)
println(x.next(), x.hasNext, y.next(), x.hasNext, x.toVector)

Expected result:

(0,false,1,false,Vector())

Actual result:

(0,false,1,true,Vector(2, 3, 4, 5))

The problem is in the implementation of the Leading class in Iterator.span, which doesn't remember that the test condition (p(self.head) == false) has already been reached.

A solution is to add a class variable var finished = false, which is set when the condition fails in advance():

def advance() = {
  !finished && self.hasNext && (if (p(self.head)) {
      lookahead += self.next
      true
    } else {
      finished = true
      false
    })
}

The implementation remains non-thread-safe.