As people noted it's because likely you have a current leakage. Read this post for more information: http://superuser.com/questions/462244/electric-shock-mild-v

As people noted it's because likely you have a current leakage. Read this post for more information: http://superuser.com/questions/462244/electric-shock-mild-vibrating-sensation-on-macbook-pro-when-charging http://superuser.com/questions/462244/electric-shock-mild-vi... I recently experienced this while travelling to India. Turns out the house I was staying at had a grounding issue and I couldn't do anything. This is less likely for most people and it's just better to use the cable with the grounding pin.