I just finally completed an absurdly tedious website maintenance project: manually correcting about 900 broken links for old bibliography items (and quite a few not-so-old ones too). I have been chipping away at this chore in tedious little batches, once or twice per week since last August. This is a perfect example of how it takes an almost unbelievable amount of TLC to keep a website like this not only running smoothly, but in good condition.
I have been collecting references for about twenty years now — see the bibliography homepage — and almost every item from the first few years had moved or vanished. Decade-old URLs seem to have about a 20% chance of still being valid. Big publishers in particular — like the medical journal publishers — seem to change their URL scheme more often than I change my socks. I’m sure many of the links I fixed already need to be fixed again!