There's a bunch on StackOverflow. I'll share a few:

http://stackoverflow.com/questions/2428404/tricky-interview-question-for-c

http://stackoverflow.com/questions/50447/favorite-c-interview-question

http://stackoverflow.com/questions/4269838/interview-question

http://stackoverflow.com/questions/2532102/can-a-pointer-ever-point-to-itself

http://stackoverflow.com/questions/3055141/interview-question-about-virtual-functions-in-c