It's funny how some questions can be harder for medium students than they can be for poor students.
A question I want to write on two-dimensional vectors was "Create a 10x10 vector and initialize it so that v[r][c] = v[r][c-1] + v[r-1][c]". I wanted them to freak out a little bit about the right way to scan through the vector before finding the right way.
The problem is, the good students will freak out, and then figure out the right way. The bad students won't even notice, and will just fill it in row by row (which works), and the medium students will realize that there's a trick, freak out, and do something wrong to fix it.
I'll probably write a modified version of the problem, but I just thought that was interesting.