Oddly enough, on my first LHR-JNB we had a bit of a medical emergency when the lady behind me started having breathing difficulty (possibly a panic attack??). Fortunately, the CSD and a passenger with medical training were able to stabilise her and they actually put her in one of the jump seats for the rest of the flight (no spare flat bed seats in F/J according the CSD). Other than that and a couple of minor medical emergency's on short haul flights, I've been relatively lucky.