Another point about flutter and the cause and manifestation:

Mass balance and mass distribution.
When building a kit or from an old drawing someone might be tempted to change a "heavy" spruce/birch wing leading edge spar to balsa that is strong enough. Later on when covering, heat shrink plastic is substituted for doped on paper.

On the first flight the wing flutter and the modell crash.

First, the shrink wrap covering is less stiff than doped on paper, making the wing more flexible in twisting motion.

Second, with a lighter leading edge the centre off mass moved aft, closer to or aft of the polar centre in the wing. (Polar centre, the point the wing will naturally twist about.) With the centre of mass moving aft there is less damping in the system untill it goes aft of the polar centre and the system become increasingly unstable. This make it easier for any disturbance to set the wing in oscillatory motion and the amplitude of the motion will increase faster.

The combination of these two changes are really bad, but the mass part might be as important as the stiffness part.