Yep, and at least here (Italy) up the '60's or maybe even '70's many older houses were "retrofitted" with what was called "piattina", which you can see here attached to a switch and to a socket:

http://www.museolaluce.com/wp-content/uploads/2009/03/vimar4...

it was two wires (no earth/ground) and was fixed using tiny nails that were driven in the plastic slip between the two wires, the "luxury" version were these (on the right):

http://www.museolaluce.com/wp-content/uploads/2009/03/btic.j...

but the most common ones had not a real insulator, they were simply steel nails dipped in some rubbery paint.

In theory the nail should have never come in contact with any of the two wires, but expecially where there was some corner preventing the nail to be hammered perfectly straight or because of sloppy installation, after a few years (with the rubber/insulation becoming stiff and microcracking) it happened.