Demonstrating how cyclic references which are both made weak will result in a GC crash.

cyclic.nim

type
  RPizza = ref Pizza
  Pizza = object
    Backref: RPizza

proc NoPizza(self: RPizza) =
  debugEcho "Pizza eaten by ghosts."

proc main() =
  var x: RPizza
  new(x, NoPizza)

  var y: RPizza
  new(y, NoPizza)

  x.backref = y
  y.backref = x

  GCUnref(y)
  GCUnref(x)
main()

GCFullCollect()