I do the same thing - 'queuing' cleric to turn off before the dps cast is completed.
I've been having this case recently also (I just chalked it up to WHM burnout) - I never really had a problem before last week. I'd hit it while it was available and start healing..... for smidgeons.... only to find off it never went off. Caused a near wipe leveling in Bray NM on my AST. >_> Second boss - I was stuck in a bubble with the DPS tunnel visioning on the boss, the tank was the only one trying to get me out (along with me) but my HP is ticking + getting a face full of dragon breath because the tank is the only one trying x_x I switched from Cleric in trying to DPS my way out to heal..... that didn't turn out so well. The tank and I in dire straights with tiny heals - and with the two of us barely getting any HP on us, I didn't have any time to look at the problem - just slap Lightspeed and spam the AoE heal to keep us both alive. The two of us lived, barely, but...... I want to strangle cleric, now, when I see it still on.
