You deployed a Next.js App Router app, called revalidatePath() after a mutation, and the page still shows stale data. Or you disabled caching on a fetch call and the page is somehow still cached. Or you migrated from Next.js 14 to 15 and everything that was working broke silently. All of these happen because the App Router has four separate caching layers , and they each have their own invalidation story. Developers hit these bugs constantly because the mental model of "there is one cache"...