Yea I know, constant vigilance and all.
but it all start with the initial build up.
If the tank can't build the hate in the most optimal time (note that I say optimal, not fastest), the fight will prolong, patrolling mob will show up, etc etc
The initial build up important, especially when doing dungeon with invulnerable, patrolling mob (yes, like Stalker in WP), in the most optimal way (note again that I say optimal, not fastest)