Detections of gamma-ray variability of active galaxies on time scales of a few minutes revealed the most extreme regimes of dissipation and particle acceleration in relativistic plasmas. Observations of blazar 3C 279 by the Fermi Large Area Telescope during a successful Target-of-Opportunity pointing campaign in June 2015 detected very clearly and for the first time variability in the GeV band on time scales 5 minutes and possibly shorter. This result presents a unique challenge for the theory of relativistic jets, since 3C 279 is also a quasar with dense radiative environment that can readily absorb gamma rays produced at sub-pc distance scales. The parameters required to explain such variability are extreme, regardless of the assumption of the radiation mechanism (inverse Compton, synchrotron, lepto-hadronic). Very high bulk Lorentz factors, Gamma ~ 100, and kinetic beaming effect of relativistic magnetic reconnection are proposed as ingredients of a complete solution to this problem that remains elusive.