Parameter variations have become a dominant challenge in microprocessor design. Voltage variation is es- pecially daunting because it happens rapidly. We measure and characterize voltage variation in a running Intel⃝R CoreTM2 Duo processor. By sensing on-die voltage as the processor runs single-threaded, multi- threaded, and multi-program workloads, we determine the average supply voltage swing of the processor to be only 4%, far from the processor’s 14% worst-case operating voltage margin. While such large margins guarantee correctness, they penalize performance and power efficiency. We investigate and quantify the benefits of designing a processor for typical-case (rather than worst-case) voltage swings, assuming that a fail-safe mechanism protects it from infrequently occurring large voltage fluctuations. With the investigated processors, such resilient designs could yield 15% to 20% performance improvements. But we also show that in future systems, these gains could be lost as increasing voltage swings intensify the frequency of fail-safe recoveries. After characterizing microarchitectural activity that leads to voltage swings within multi-core systems, we show two software techniques that have the potential to mitigate such voltage emergencies. A voltage-aware compiler can choose to de-optimize for performance in favor of better noise behavior, while a thread scheduler can co-schedule phases of different programs to mitigate error recovery overheads in future resilient processor designs.