Yeah, it's one of those things that people will disagree on. The main problem was something to do with fibre being counted as 0 calories when it's actually 1-2 cals per g (or something).
I should imagine that celery possibly falls under this catagory, being solid and fibrous, and with so few calories, you probably burn more chopping it with a knife, and that's before you start digesting it
I can only imagine these are low cal foods that use more calories to chew and digest than they originally contain, thus taking you into negative calories. I might be wrong though