Accurate motion perception is critical for successful interaction with the environment. We report that monovision, a common prescription lens correction in use by millions, can cause people to dramatically misperceive the distance and three-dimensional direction of moving objects. With monovision, each eye is corrected to focus light from a different distance, causing one image to be blurrier than the other. The misperceptions occur because the blurrier image is processed faster than the sharp image. The mismatch in processing speed causes a neural disparity, resulting in illusory motion in depth. The illusion, a variant of a 100-year-old stereo-motion phenomenon, poses an apparent paradox: blur reduces contrast, and contrast reductions are known to cause neural processing delays, but our results indicate that blurry images are processed milliseconds more quickly. We resolve the paradox with known properties of the early visual system, show that the illusion can be severe enough to threaten public safety, and demonstrate that the misperceptions can be eliminated with novel combinations of existing optical technologies. The fact that substantial perceptual errors can be caused by millisecond interocular differences in processing speed highlights the exquisite temporal calibration required for accurate perceptual estimation. Our newly discovered motion illusion—the ‘reverse Pulfrich effect’—and the paradigm we use to measure it, should prove useful for understanding how optical and image properties impact temporal processing, a fundamentally important but understudied issue in vision and visual neuroscience. Directions for future work will be discussed.