Warrior's endure pain has been bugged for a very long time now. The bug consists of the following:
This bug is noticeable when fighting large groups in WvW due to the fact that most players play necromancers and revenants whose attacks deal life siphon damage and in the case of necromancers who can share buffs with others to allow them to siphon live as well. Thus, when a warrior uses endure pain in large fights, usually it's ineffective.
This bug is also noticeable when fighting revenants who use Assassin's Annihilation.
A revenant can easily melt through endure pain by using Assassin's Annihilation, thus rendering endure pain useless.
Please, fix this breaking bug before PoF.