Having trained and worked over many years in analysis I can say that the objective of the analysist, through hypnotic (doesn't have to be hypnosis)free association, is to actually seek and find the cause of the effect and guide the client INTO an abreaction.
They then say that the bubble has popped and all is permanently fixed.
However, many analysts, over a few drinks (it takes quite a few) wil eventually confess that over many years they have never witnessed an abreaction!
Make of this what you will.

Well said Bob. I've had some pretty strange things happen on stage. Unexpected tears twice, uncontrollable laughter many times - but that's usually me - and once and only once a young girl who spent the whole show 'talking' to her recently departed grandmother on the astral plane. And very happy she was about it too.

I think most hypnotists wouldn't know an abreaction if it got up and bit them, and for most it would.

If you are using direct suggestion properly on stage there should be no chance of anyone doing anything unexpected really, at least nothing you are not prepared to easily handle.