A few years ago a co-worker was given the task of installing a replacement PC in a university lab. This was a relatively routine task that he had done many times before, and basically involved swapping the new machine out for the old one. He ran into a bit of trouble when he couldn't get the new machine on the network, and after some back and forth troubleshooting realized port security had likely been enabled on the switch port to which the machine was connected. Thinking creatively, he got on Google and figured out how to change the MAC address in Windows to another to the MAC address from the old PC, and got the new machine on the network, told the user his PC was ready, and closed out the ticket.
Was this a case of great customer service getting a PC into the hands of the user right away, or a horrible idea? I can see people arguing both sides, but I'm going to go with the latter.
The tech in question probably did not want to deal with the bureaucracy of getting port security disabled, or the MAC address changed on the switch. It's also possible he didn't know who to contact, or what process to go through, and didn't want to admit it. Or, he had recently learned of the particular registry key that was changed and thought it'd be neat to try it.
The problem with a quick hack fix is that it almost always bites you later.
If someone was to plug the old PC back in to the network at another location (which often happens when old machines get sent to surplus and then get reclaimed), there will be two machines with the same MAC address on the network. This is bad.
If someone were to reload the operating system on this PC, the hack that was put in place would be lost, and they'd have no idea what changed or why networking started to fail. This could lead to hours of troubleshooting, especially since port security wasn't something that was commonly used in this building.
Worse, no one else in the IT support group knew this was done since it wasn't documented anywhere.
Lastly, this was a complicated solution to a relatively simple problem. As a rule, I try to avoid touching any knobs, buttons, dials or other configuration settings unless there is a good reason to change them. Fixing this particular problem at the source would have meant fewer configuration settings overall (a configuration option on the switch port, and then another configuration option attempting to override it locally on the PC versus having it set up correctly).
What ended up happening? About a year later, the PC in question got compromised with malware and had to be taken off the network and re-imaged. I just happened to overhear a co-worker mention how he couldn't get it back on the network and wondered if the NIC was broken. Because I have a disturbingly good memory I mentioned what I had been told about the machine when it was installed, and he got the networking group to fix the switch port.