I have played multiple characters since November of last year and have done the quest http://www.uesp.net/wiki/Skyrim:The_Man_Who_Cried_Wolf numerous times. Last night
I experienced a first. I was standing on the outlook at the end of the cave. I am playing a Ranger (level 36) and I still have Lydia alive and by my side. I was looking at the keep with the blue swirling light and started to let fly the arrows at the targets visable on the keep platforms and the grounds below. I could slightly see the necromancers at the top of the main building so sent a few arrows that way as well. I all of sudden got the message that I finished that quest and needed to return to Solitude for my pat on the back. I looked back to make sure Lydia was still at my side and had not gone solo into the keep, she was still there. I went down and inspected the rest of the cave. Except for one lone draugr in the back of a passage way inside the keep they we all gone and ash piles were everywhere with a littering of the undead. Either the game decided that I was good and could go home or I landed several increadable shots. Has this happened to anyone else on this or any other quest?

Yep, did this as well, sniped everything except three draugr from that ledge. When you kill one, the others come, and into my range

Edit:Seems they fight against each other as well, 'cause the necromancers killed half the draugr for me.

EPIC

i use this quest to raise my archery skill by doing exactly what you did. and by crouching at the same time, also raises my sneaking skills.

I also usually only miss 1-2 hidden away.