A realization about breadcrumbs
Our internal user support web pages don't currently have breadcrumbs, those little bits that show a web page's location in some hierarchy and let you pop up to higher levels. This makes a lot more sense than for most websites, because hopefully we don't have our users landing on random pages through search engines; in theory they come into our support pages at the top and then navigate down from there, so they always have the Back button available.
Then, recently, someone had a question that was best answered by one web page, except that the answer could be expanded by a parent page and a sibling. And I got a sinking feeling.
Although it should have been obvious to me earlier, clearly search engines aren't the only way that our users can wind up on random support pages instead of the top of the tree. And it's not just the URLs we give them to answer their questions; users may well be passing URLs around without involving us, as one grad student asks another for help.
(I tend to assume that this happens a lot, but that's another entry.)
Or in the short version: breadcrumbs mean you can give out any of your URLs, knowing that people can navigate outwards from there to whatever else they need.