"Self-publishing companies"

In the sense that many people are using this term these days, it's a misnomer. Self-publishing companies are what self-publishing authors start and operate to publish and sell their books — not iUniverse, Lulu, Author House, or the like.

More than just splitting hairs, this can be an important point of contention. For example, if the "self-publishing company" owns the International Standard Book Number for the author's book, then that company is actually the publisher of record; technically, the author isn't even self-publishing. Worse yet, some of these companies use the term specifically to mislead authors, usually to cover for their actually being a vanity press.

So, consider this little semantic distinction the first self-publishing mistake to avoid. Instead, call these companies what they are — self-publishing services, print-on-demand publishers, subsidy publishers, or whatever is most appropriate. For more on this, read Andrew's e-book.