Flutter and Buzz
Flutter usually runs away to destruction and is the bane of the test pilot. Often bonkers/exciters are fitted for flight tests near the speed boundaries to induce oscillations as forewarnings.
Modern design computer programs can predict most flutter situations as another giant leap along the track to enhanced safety.
Control surface buzz is usually transitory but should not be tolerated, often an indication of loose bearings, linkages, weakening ribs and the like.