Arcing or sparking at a point of electrical contact usually means the prong or pin is a poor fit for the outlet. The problem could be your adapter's male pins or prongs being bent, worn, or slightly too small, or else the female parts of the wall outlet being likewise worn. This sparking will cause blackening or charring and generates heat. You may notice a smell of burnt plastic. The overheating could damage or destroy the adapter and even start a fire. If it only does it at one outlet, avoid that outlet. If it does it all the time, discard the adapter.
