unless we're talking about Lost Isles

With regards to the limits of phasing, it phases small areas, not entire zones (unless we're talking about Lost Isles, Kezan, or Gilneas). It should never be a problem--just leave the phased encampment or progress by yourself. Phasing is never used with regards to elite bosses while questing--at least none that don't come with the right quest tools.

With regards to small areas, acheter wow gold I've noticed this particularly surrounding the initial town (known for harvesting a special brand of "seedling" -- further details would be spoiler territory) of Hillsbrad. After completing the questline, if you enter the town's area, it automatically phases you from the world. How can you tell? Well, the deer mobs surrounding the town actually disappear.