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Recently, a new dynamic voltage scaling (DVS) scheme has been proposed that increases energy efficiency significantly by allowing the processor to operate at or slightly below the minimum supply voltage, even if occasional errors result. To determine which technique can reliably and efficiently detect such failures, it is necessary to understand the manner in which digital designs fail at critical voltages. We report hardware measurements of the failure modes of a multiplier circuit under voltage scaling. We show that even at small error rates, it is necessary to deal with multiple errors where bits are flipped from both 0 to 1 and 1 to 0. Intra- and inter-die variations make the exact nature of these flips unpredictable. This suggests that conventional single and unidirectional error detectors will not work. We conclude that the most suitable solution is a simple delay-error tolerant flip-flop that detects and corrects errors by double sampling signals.