A long time ago i ran an underdark campaign where the party was roughly 8th level, then they recruited three 12th level NPCs to assist them. I'll never let that happen again. Long story short, the party was tactically sound on all fronts, and i tried EVERY adventure to put them in situations to kill them, simply because they always escaped. Hurt - yes, but if i didn't pull tricks out of my sleeve the danger quotient just wasn't high enough. I never did actually end up killing anyone, except for at the end from a suprise fiendish red dragon attack at close range. Then the dragon died two rounds later...
I think the problem was that the party had very smart players, and with all the PC's and NPC's and animal companions, there were 8-12 of them on the board at any given time. They flat-out outsmarted and out-strategized me with surprise abilities and spells, so i had to get dirty to keep the tension amped, otherwise it would be a cakewalk for them.