What Paper is (and Isn't) Good For - Paper Prototyping: The Fast and Easy Way to Define and Refine User Interfaces - page 213

Finding Problems through Inspection
Some types of usability problems can be found simply by looking for them—the false top, cascading
menu, and keyboard-mouse issues are ones I can often predict just by looking at a screen. However,
there's something of a Catch-22 here: To find a problem by inspection in a paper prototype, you have to
know to look for it and that knowledge only comes from having seen examples of it in real interfaces
and/or interface guidelines. To the extent that you're able to educate yourself about common usability
issues, you may be able to use that expertise to partially compensate for some of paper prototyping's
deficiencies. But don't expect "armchair usability" to replace the real thing because there are many,
many important problems that you'll never find if you look only for the problems you know about. (If you
want examples, reread Chapter 2 .) Usability testing is useful way to check your blind spots.