  The fluctuating superconducting correlations emerging in dirty hybrid
structures in conditions of the strong proximity effect are demonstrated to
affect the validity range of the widely used formalism of Usadel equations at
mesoscopic scales. In superconductor -- ferromagnet (SF) structures these giant
mesoscopic fluctuations originating from the interference effects for the
Cooper pair wave function in the presence of the exchange field can be
responsible for an anomalously slow decay of superconducting correlations in a
ferromagnet even when the non-collinear and spin-orbit effects are negligible.
The resulting sample-to-sample fluctuations of the Josephson current in SFS
junctions and local density of states in SF hybrid structures can provide an
explanation of the long range proximity phenomena observed in mesoscopic
samples with collinear magnetization.
