Electric shock from APC UPS

My APC 500 Va Back ups caused the computer back unit to give me an electric shock. This happened when the UPS was switched off. All the users of Computers may stick to the age old custom of shutting down the mains supply before touching an electrical gadget like a computer and never allow kids to touch metal parts of UPS connected computers, a lesson which APC made me re-remember in my 40th year.