In accordance with my complete lack of interest in what constitutes "fashion", I have apparently been committing a possible fashion no-no for years, in the form of the "Double Denim". This is a denim top coupled with denim pants. I, of course, take it one step farther by wearing the denim shirt under denim overalls (albeit hickory striped, in this case). But in general, I fail to see what this woman is doing wrong by pairing denim with denim...or this one (dance step aside)...and then there's...well, you get the idea.
Generally, I like the way denim feels, so why not pair it? Double denim rules!
UPDATE: I would later don double denim -- this same denim shirt with another pair of overalls -- for my 40th-birthday pies in the face. Double denim still rules!