One guard rewrote the file every public link reads, and the host it wrote was already dead.

A guard process overwrote the single file that every public URL in my system is built from, writing a quick-tunnel hostname that changes on every restart. Twenty minutes later that host answered nothing while the product, the stable host and every local probe were fine. The type check that stops it, the eight-case test whose first run failed on a whitespace-only string, and why a liveness probe cannot substitute for a name that survives a restart.