Publishing a post without any category assigned to it (e.g. after unselecting the General category and forgettting to select another) results in a rather cryptic error messsage referring to a
foreach() statement in
template-functions.php. It took me a while to figure out the reason, and it was only because I could understand what the
the_category function does.
A simple way to make the whole thing user-friendlier would be to a add a short feedback message, perhaps along the lines of "e;You must assign at least one category to every post"e;.