There are only two hard problems in computer science: cache invalidation and naming things.
That's how I learned the phrase.
There are only two hard problems in computer science: cache invalidation, naming things and off-by-one errors.
Keep track of all items (like "main page", "search results for $blah") that component items (like "body of post 234", "avatar of user 3540") are used in, and when a component item changes, invalidate its cache and that of all items it was used in.
I'm pretty sure the quote was originally talking about function, variable and data structure naming, in which Google may or may not be help. Really, I've always found naming problems in that are more to do with correctly predicting the future, since the most egregious problems I encounter are when something that used to be named at least passably has changed over time to be very confusingly, if not outright misleadingly, named.