From my limited aerobatics experience, I was taught that you cannot spin the airplane from a slip, but only from a skid. If stalled while in a slip, the plane will do a "falling leaf" maneuver. Quite a weird feeling to hold it in one, but the falling leaf is pretty unstable and can oscillate to a skid pretty easy and start a spin if you're not careful.

Being experimental, I've tried slips in a 182 with 40 degrees of flaps. It pitches the nose down pretty violently. Before I tried it, I heard in a lecture, at an ESP CFI seminar, that the violent nose pitch is the reason planes are placarded for no flap slips (he was an FAA examiner so I'll consider it a credible source).

And that knife edge maneuver is awesome!