Were you using an official Apple charging cube? Or a replacement charging power supply? Many third party power supplies, including ones that look exactly like an Apple device, are inferior when it comes to safety. In particular, these imitators do not obey the rules that specify the separation between the highvoltage components and the low voltage side of the charger (the part that potentially comes in contact with your hand via the case). For some hair raising observations about the quality of non-Apple chargers, see this article on Ken Shirrif's blog, "iPad charger teardown: inside Apple's charger and a risky phony:"
http://www.righto.com/2014/05/a-look-inside-ipad-chargers-pricey.html
The problem you were originally describing is almost certainly a function of the charger and not the iPad itself; even if it was a failing Apple charger you could probably put the matter to rest by going to an Apple store and buying a new charger (don't trust buying one from Amazon - they're stuffed with counterfeits!).