This was quite recently, actually.
I've been modding Skyrim to hell and back, and one of my favorite mods was Dragon Combat Overhaul. It alters the AI of dragons to make them smarter and tougher.
Anyway, I was headed up to Northwind Summit to learn the shout from the word wall up there, when I was (unsuprisingly) attacked by the dragon guarding it. It was a standard dragon, not to difficult to fight. Except this one was different. It took me a good six attempts to beat it.
First Attempt - The dragon caught me by surprise, landed on me, and sent me tumbling down the mountain. Game over.
Second Attempt - I got it down to half health, but didn't realize I ran out of health potions. He eventually outlasted me and roasted me to oblivion. Game over.
Third Attempt - I'm frustrated and impatient at this point, so I charge in head on. Bad move. The dragon used its triple threat tail attack to ragdoll me to the ground. It spewed fire at me at close range, and since I forgot you could use potions while ragdolled, I died. Game over.
Fourth Attempt - I finally lower the difficulty to Novice out of sheer spite for this scaly beast, but that didn't do much. One triple threat tail attack and I'm sent ragdolling over the edge of the mountain to my death. Game over.
Fifth Attempt - I'm fuming, and I'm starting to have thoughts of uninstalling DCO just to have a chance at this beast. I got it down to half health again, but I ran out of health potions...again. He outlasts me and kills me yet again. Game over.
Sixth Attempt - After a quick break to calm my nerves, I got back into the game, ready to finally kill this thing. Somehow I did. Through sheer luck and brilliant tactics involving a bow and arrow, Dragonrend, and escape tactics, I finally bring the dragon down. I'm rewarded with the Alduin death animation.
I was so proud of myself, I decided to take a screenshot of me finally killing it.