Another Felix

Fizz's E and Zhonya's Hourglass are two examples.

Basically, if you are untargetable, you are fine. ((or have a Spell SHield, like Sivir's E.)) otherwise... no it always hits. Banshee's Veil is usually a pretty good option to consider when Karth is in a game ((it is period... but even more so with him floatin' around.)) because it will block his ulti. It will also allow you to disregard the first spell of an initiation by the enemy team as well.

Kill or CC Karthus