I've been encountering a few mesmers a day who bind mantra of pain, f1 and GS 3 to all cast upon hitting one button. Now I understand that mashing all 3 buttons at once can almost have the same effect, but still leave a delay long enough to dodge or use any defensive CD needed to survive the burst. Recently I've been noticing a hand full of mesmers a day using what is basically an exploit to remove what little counterplay there is to their burst.
You can tell when a Mesmer is using this exploit by the sound of GS3 and mantra of pain happening at literally the same exact time. I've played against enough GS power mesmers to understand how quickly that combo can legitimately be casted.
I can provide video proof if needed or you could watch any streamer playing in a pvp environment