What happens if you touch a patient while using the aed?

Starts over. The most concerning issue would be touching the patient at the time the AED delivers a defibrillation. The electrical current could pass from the patient into anybody making physical contact, and cause various injuries or even stop your heart. Because of that, most aeds will stop their analyzing cycle if somebody touches the patient, and then start over with analysis prior to advising a shock.
Nothing dangerous. If you feel anything, it will likely only be a slight tingle at the time of the shock. Although it is recommended that there be no contact with the victim when a shock is delivered the danger of receiving a significant shock yourself is negligible.