When I started in this business I was blessed to apprentice under three reputable brothers. I learned the trade from each of the siblings, each with their expertise in gold smithing. They did not cut corners and took pride in their work. It was not until years later when I saw another side of the business. Like in all vocations there are bad apples. (A very small minority) We have all heard the horror stories. But I am not opening up that can of worms.
A minor offense is using low karat to repair chain and soldering in heads. The lower melting temperature makes it an easier repair. It makes you feel like a super repairman. The down side is that the soldered area tends to tarnish. It is also a less expensive solder. It does not seem like a good place to cut corners. But we see it from time to time.
Here is a peg head soldered in place with low karat solder. This darkness at the base will repeatedly come back even after repeated polishing.