From The Heretic Knowledge Vault
An Errant is a sad subclass of Half Elf, but there's more to it than that. The historic meaning of the term "Errant" confined it to describing half elves who had congenital defects.
The Errant Problem
The genetic merging of elf and human doesn't always produce perfect results and sometimes there are aberrations. These are called Errants, a less than perfect genetic merger that results in offspring with congenital birth defects that range from sterility to psychosis. While the physical aberrations are obvious and relatively blatant, the mental aberrations are somewhat less so and their onset can come without warning or they can manifest themselves in extremely subtle ways.
This is where the form persistence that they inherited from their elven line went from benefit to curse. Due to this persistence, they couldn't be cured of their congenital problems by either magic or medicine.
This only applies to actual internal changes to how the elf/half elf’s body is made or works, its natural genetic structure and the like. Incidental injuries, like cuts, broken bones, bruises are a completely different matter. This also includes bacterial and viral illnesses since those don’t really alter the body’s inherent make up in any way, normal healing affects those just fine. However congenitally caused illness, schizophrenia, depression, psychosis, among others, as well as outward physical defects are impossible to cure among Errants, as elven Form Persistance eventually returns the Errant's body to its natural form.
The Fundies amongst the Elves
After several spectacularly destructive incidents (Notably the fall of Malacia, followed by the disappearance of Tsuiraku a year later), fundamentalists of the old Elven Religion began a campaign of moral outrage and fear of these Errants. They pushed for extermination, a genocide as a mercy-favor to their half elven children. This eventually lead to the Errant War, driven by religious paranoia and outrage.
Through the fundamentalists the term Errant crept into common elvish usage as a derogatory term for all half elves. Only elves refer to Errants. Humans and half elves use the more readily comprehensible half elf.
Sarine, as usual, goes her own way in dealing with Errants, even though her job as a Ranger/Peregin is to patrol the world beyond the elven lands and eliminate threats to elvenkind -- which most elves take to include all half elves, whether defective or not. Her lengthy monologue in Chapter 10 is responsible for much of what we know, or think we know, about the state of Errantcy. Most of the other elves we've seen (Sarna, Toren, Elven Council, etc.) aren't nearly as careful to distinguish between "normal" half elves and Errants, which is one of the main reasons we have a story. Note, however, that Sarine's friend Misa seems to understand that a half elf is not necessarily an Errant. Well, at a mere 1400 years old, she's too young to know any better.
- See Poe's notes on Healing