Stroke within 24 hours of neck manipulation has been reported. It's not common. There are an equal number of cases of stroke in patients treated with nonforce in a neutral position. In other words the neck isn't rotated or extended during the treatment.
Risk of serious complications after neck manipulation is being studied. A recent report from California shows that mobilization has fewer side effects than manipulation. Mobilization is a movement of the joint through part or all of the range of motion. There is no thrust and no high velocity motion.
Studies estimate the chances of stroke after neck manipulation range from one in 500,000 to one in six million. Based on malpractice claims it appears the risk is very low.