People who can't explain how current, voltage, and resistance relate to each other shouldn't be allowed near wiring.

A fluorescent lamp that one would attach to furniture probably only takes 10W. That's less than 100mA at 120V; a telephone wire can handle that. Also, telephone wires once carried as much as 50VDC for ringers, so I'd expect the insulation to be adequate, if only barely. The biggest problem, I think, is that because the insulation is relatively thin, there's a greater risk of mechanical damage to it.

One slightly obscure example is someone using a European (220V) light fixture or extension cord (with plug adapters) in North America. (110V).

The current for the same wattage will be roughly double in North America. So lamps, etc. intended for use in Europe will have wiring that is undersized for the current draw if used in North America.