Related to this question.
Our gas furnace, while quite functional, has some quirks in its installation. One of them is that the installers cut off the old (original to the house) thermostat wires at the ceiling above the furnace and open-spliced (with wire nuts) new thermostat wires to the old. While not all that neat and workmanlike, this is Code-legal for class 2 control wiring as far as I understand it. There is also one point where the wires are attached to the EMT feeding power to the furnace, which is covered by 300.11(B)(2) in the 2014 NEC.
More interestingly, though, the majority of the support for the new thermostat wire is provided not by the EMT for the furnace power, but by the black iron gas line feeding the furnace. Yes, that's right: the furnace installers neatly zip-tied the thermostat wire to the gas line at regular intervals. Is there anything in the 2006 IRC/IMC/IFGC, 2002 NFGC, or 2014 or 2017 NEC that prohibits this?