Major character who has only appeared on 1 page in the entire OOTS saga

[Goblin_Priest]

I think the argument that Rich egregiously miscounted something and put that in a published book, or that he used a bunch of technicalities to dismiss appearances until he arrived at "one," doesn't make prima facie sense.

I don't know. I'm not saying I think it happened... but... if it did, it wouldn't be all that extraordinary. Look at all the plotholes we find in movies, which are overall pretty short works reviewed by countless people. They obviously also slip a lot into books, given these are longer works managed by fewer people. This comic? Is enormous volume of content, managed by a single person.

Not every artist is an accountant at heart, keeping detailed spreadsheets of every single character and events.