Types of Difficult Physics Problems

In my years of teaching physics, I've noticed that the truly difficult, student-stumping problems almost all fall into one of the following five categories:

1. Conceptually hard - "I don't know what the physics is!"
   
2. Mathematically hard - "I don't know how to solve that equation!"
   
3. Nitpicky - "I can't find where I lost that factor of x!"
   
4. Misread - "Wait, I wasn't supposed to find the impossible variable, I was supposed to find the easy one?"
   
5. Paranoid - "I'm not leaving until I find the trick to the question...wait, it's not a trick question?"
   

Obviously, #1 and #2 can only be overcome by learning more. But a lot of "hard" problems fall into the other three categories, and can be solved simply by being careful when reading the problem and when doing it...and by not looking for problems where there are none. :)