Next.js 16 caching, PPR, and streaming bugs causing HTTP 500s, hydration errors, and silent router failures

Developers using Next.js 16 report multiple critical caching, partial prerendering (PPR), and streaming defects. These include unstable_cache past expiration causing HTTP 500 responses due to skipped resume data caches, server action POST requests on stale PPR shells re-running and failing on already-consumed request bodies, layout "use cache" hooks re-executing at request time causing React hydration error #418 due to HTML/RSC payload mismatches, background regenerations aborting on Vercel because they aren't kept alive via waitUntil, and router.refresh() silently failing on streamed pages.

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