[quote][Contrary to some comments above, a gap does NOT cause flutter. It does lower the response (the smaller the gap the more control you have for the same amount of physical movement) rate. A gap could initiate flutter by stimulating the resonant frequency of the surface. Every thing will flutter if given the proper stimulus. Ideally the only surface that will not flutter is a surface of zero mass and infinite stiffness, a totally unobtainable ideal so the best we can do is make the surfaces as light as possible yet retain a high degree of stiffness. Most sport models will fly perfectly well with very large gaps between stab and elevator but can be somewhat sluggish if the gap is large.
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I have to disagree with the above. I have been flying RC for more than 40 years and have seen a number models with flutter. Anything that gives control surface slop will increase the probablity of flutter significantly.

Most airframe examiners that I know would not allow a model with a 3mm control surface gap to fly.