There was only the remotest possibility of him having the least degree of trouble on such familiar spells, but to have the bad luck to have failed on two of them in short order?
That troubled him greatly.
Suspecting something, he was not a great genius for nothing, Dumbledore quickly scanned himself. When he found what he did, he quickly turned and, in a rage, cast a Crucio on Severus, holding for several minutes until the man had gone quite mad.
Then, having worked out his rage, he Obliviate the memory of that experience and told Snape curtly, "You failed to check for residuals, my friend. Not only did some enemy slay me, they also injected malaclaw venom into my body."
Curled up on the floor trembling and weak as his body spasmed from the aftereffects of exposure to the Cruciatus, Snape could only simper weakly that he was sorry.
Sighing, Dumbledore dismissed the man from his concerns and summoned a bottle of the antidote from his office, only to have the wards report that it smashed on the inside entrance to his tower, the door having failed to open.
Cursing himself for having forgotten his luck, Dumbledore stormed up to his tower himself to fetch a bottle personally, slipping on two banana peels before he'd even left the Great Hall, cracking his head each time, and running into countless faces he'd rather not see on his way there.
Partially Kissed Hero, Chapter 32