The U-M Experimental High Altitude Long Endurance (X-HALE) aircraft is an unmanned testbed meant to provide flight data of aeroelastic phenomena for the validation of nonlinear aeroelastic simulations. It was designed with an unstable dutch roll mode that couples with the first wing bending mode - seen here in three different configurations.
Note that each successive configuration change (clockwise from top left) would have increased the stability of a conventional aircraft, so this video demonstrates that traditional modelling and control techniques are insufficient to predict and suppress instability in very flexible aircraft like X-HALE.
Publication: Jones, J.R., Cesnik. C.E.S. "Preliminary Flight Test Correlations of the X-HALE Aeroelastic Experiment." The Aeronautical Journal, Vol 119, Issue 1217

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