Recent evidence has identified a risk of neck injury resulting from exposure to high sustained G force during operation of fast jet military aircraft. This review considers the risk of acute soft tissue neck injury when exposure to acceleration is unexpected by the individual. In a military jet, this might occur to a back seat occupant such as an instructor or weapons operator/navigator, but unexpected forces may also be encountered by roller coaster riders and human centrifuge users under certain conditions. The mechanisms of injury in this situation are examined, the effect of acceleration axes (Gx, Gy, and Gz) are considered, and relevant experimental data are evaluated. These data are used to determine what influence the unexpected nature of the acceleration exposure has on the risk of neck injury. Injury below previously established 'tolerance' limits is possible, with some individuals being at risk from unexpected acceleration exposure above around +2 to .