Also at FB: one day we got a huge spike in measured site-wide cpu usage. After the terror subsided, we found that a single request on a single machine had reported an improbably huge number of cycles (like, a billion years of cpu time). We figured a hardware problem and sent it to repair. A month later the same thing happened to the same machine; it had just been reimaged and sent back into the fleet. There was some problem with the hardware performance counter where it randomly returned zero, but after that we made sure it was removed permanently.