I am doing a study in school for computer science. My research topic is pen testing, since that is what I hope to do after I graduate. While doing research I am finding things that can go wrong, but I don't know how frequently each problem is encountered.
Edit: I am very new to the subject so I hate to say my knowledge is limited. The subjects that sound the most interesting to me are OS, networking, and social engineering (but I think that is a different can of worms.)