I've mistaken caffeine withdrawal for a hangover before, partly because of similarity in symptoms (headache; nausea; photosensitivity) and partly because it tends to show up around the same time (Sunday morning). This may account for the popularity of coffee as an alleged hangover cure.
Which raises the question, if the things people say about "hangovers" are true about the things they apply the term "hangover" to, what's left to be debunked?
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.