If there is a ghost on the ground that isn't a buildable item (and thus not placeable by bots), the Ghost Scanner will still export a signal for it. Most notable are the "Logistic Train Stop Input" (lamp) and "Logistic Train Stop Output" (combinator).
Anything that can't actually be crafted should probably be excluded. They can't be filtered out because there is no way to set those object-signals in a constant combinator. If these signals make their way to something like a filtered stack inserter, that inserter will effectively be dead. And if construction is waiting for that inserter to provide items, it's deadlock.
-- Brian
