Wobbling
From Kerbal Space Program Wiki
Wobbling occurs when a structural joint is unstable and cannot support the forces acting on it. The part wobbling will clip through the craft occasionally, and may break off if the stress is too great. This issue is usually rectified with EAS-4 Strut Connector.
Wobbling is usually visible by parts oscillating with high amplitudes: The part is moving a great deal in any direction although it should be fixed.
