Circumstantial ad hominems occur when someone's argument is dismissed because he/she appears to have a conflict of interest. While it is true that he/she may have a conflict of interest, that doesn't necessarily mean that his/her argument is without merit.
Example: Your argument that the United States is the best country in the world is false because you're from the U.S. and have to say that.
Example: Of course Mark Zuckerberg believes the government should pay to make sure that every U.S. citizen has an internet connection; afterall, he owns Facebook and stands to make a lot of money if this happens.
Does this mean that Zuckerberg's argument is inherently bad? Not necessarily. It should be taken on its own merits regardless of whether Zuckerberg stands to make a lot of money from it or not. Frankly, because Zuckerberg really understands the uses of the Internet, he might actually have some ideas worth listening to.
Example: This article, HERE, addresses circumstantial ad hominems.