  We investigate collective flavor oscillations of supernova neutrinos at late
stages of the explosion. We first show that the frequently used single-angle
(averaged coupling) approximation predicts oscillations close to, or perhaps
even inside, the neutrinosphere, potentially invalidating the basic neutrino
transport paradigm. Fortunately, we also find that the single-angle
approximation breaks down in this regime; in the full multiangle calculation,
the oscillations start safely outside the transport region. The new suppression
effect is traced to the interplay between the dispersion in the
neutrino-neutrino interactions and the vacuum oscillation term.
