Re: using the wrong kind o' magic
It happened during World War Hulk.
He was desperate enough to take within him the essence of Zom, someone with the potential to be more powerful than Dormammu and Umar. It takes beings like Eternity and the Living Tribunal himself to subdue and dissipate much of the demon's power and banish him.
Already faltering, Dr. Strange corrupted himself further. While the Hulk beat the weakened Zom out of his system, we do not know how much of Dr. Strange's might was taken by the demon when he fled the host body. To give you an idea, it's like adding a drop of dark ink to clear water. Sure, you may be able to filter it out, but unless you take drastic measures, the ink will still leave traces behind. And Dr. Strange hasn't taken drastic measures yet, since Zom feeds on magic.
There have been Sorcerer/Sorceress Supremes before Dr. Strange, there will be after. I think this may be a great point for Dr. Strange to pass on his legacy, as it was passed on to him.