Chaining multi-socket extention cords, can it be done safely?
The wires themselves are not the problem. Humans are, because it is easy to overload the circuit, and even if it is below the safe margin of the fuse tripping or wires burning, you forget contact resistance and loose connections.
When given enough use, the contacts come loose and they oxidize and the contact area becomes smaller and contact  resistance increases. Thus the contacts heat up more and given bad enough connection, the heat starts to melt things and cause fires.