Just started exploring Northrend and have noticed that the guide often moves on when you discover an area nearby the one you're supposed to be discovering. In Borean Tundra, it's done this for almost every step.
E.g. Discovering Frozen Reach, which is not needed for the achievement, causes the guide to move on from the "Discover Death's Stand" step.
First time through Borean Tundra the guide said I had completed "Explore Borean Tundra" but I was nowhere near done.
Edit: Sholazar is even worse - it's marking areas as discovered when I'm over the other side of the zone. I did all this on an Alliance toon and it was fine.
E.g. Discovering Frozen Reach, which is not needed for the achievement, causes the guide to move on from the "Discover Death's Stand" step.
First time through Borean Tundra the guide said I had completed "Explore Borean Tundra" but I was nowhere near done.
Edit: Sholazar is even worse - it's marking areas as discovered when I'm over the other side of the zone. I did all this on an Alliance toon and it was fine.