If the Primary circuit has current flowing the Secondary Circuit should never be opened.  This can cause very high voltages to occur due to the Ampere-Turns of the primary that start magnetizing the core. This results in the core being Saturated through a large portion of the cycle. When a transformer core is Saturated, it is a non-transformer. 

Consequently, the transformer acts as a transformer only during a short period above and below the zero to maximum in the opposite direction. While it is acting as a transformer it will cause very high voltage peaks. The voltage may not appear very high taken when the measurement is taken with an RMS Volt-Meter but a Peak Volt-Meter would indicate a much higher peak value. The peaks can be sufficient enough to cause death.