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I wasn't being serious in that post, haha. It's alright, I'll just scrap the idea I had. It's not really important anyway...

What are you trying to achieve here?
Working against the engine will never end up well.

Looks like I'm going to have to fill all of my levels with dummy 3D ceilings then... :S

This isn't randomization but simply movement clipping against the ceiling plane. Unfortunately not really fixable without making an utter mess. I tried that years ago and the result was even more problems.

Even with this flag applied, the spawned puff is still jumpy when aimed at the ceiling. Same behaviour regardless of normal ceilings or 3D floor ceilings.

HOWEVER, the jumpiness doesn't happen on floors (real floors and also 3D floors). Please don't close this as [Not a bug] because it clearly isn't working as expected (happens on ceilings but not floors).

EDIT: Actually it only happens with normal ceilings. 3D floor ceilings do not exhibit this problem. File updated with a simple map to easily observe the behaviour.

https://dl.dropboxusercontent.com/u/47355875/hehpuff.pk3