  Extremely large, rare events arise in various systems, often representing a
defining character of their behavior. Another class of extreme occurrences,
unexpected failures, may appear less important, but in applications demanding
stringent reliability, the rare absence of an intended effect can be
significant. Here, we report the observation of rare gaps in supercontinuum
pulse trains, events we term rogue voids. These pulses of unusually small
spectral bandwidth follow a reverse-heavy-tailed statistical form. Previous
analysis has shown that rogue waves, the opposite extremes in supercontinuum
generation, arise by stochastic enhancement of nonlinearity. In contrast, rogue
voids appear when spectral broadening is suppressed by competition between
pre-solitonic features within the modulation-instability band. This suppression
effect can also be externally induced with a weak control pulse.
