Was it updated in recent history or something? I'm not aware of any, but Holminster Switch was always one of those dungeons that made you put everything you've learned to the test. Tanks need to know how to pace out their cd's, healers need to know when to focus heal and when to damage, and dps need to have learned their rotation. If any one falls short in either of these aspects, then it's best to take it slow. Even then, small pulls can still hit hard.

Heck, even the dungeon right after it will make you have to pay attention, otherwise the enemies will be buffed and will hit harder. It's common for tanks to use their Invuln on the first big pull because they don't realize they need to use Interject.