Cache invalidation isn’t hard, but it used to be and now we can’t get people to think otherwise.

I heard that the two main problems in computer science is that we have only one joke and it's not very funny.

cache invalidation is not hard indeed, but people always forget about it (so caches aren't caches aanymore, they become stale copies of things...). People tend to forget that a cache has to have a fixed size and an eviction policy.