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A detailed analysis of the projectile-like fragments detected at backward angles in the reactions 16O, 32S+208Pb at energies below the fusion barrier is presented. Excitation functions corresponding to nucleon transfer with ΔZ = 1 and ΔZ = 2 were extracted, indicating surprisingly large absolute probabilities at sub-barrier energies. A comparison of 2p transfer probabilities with time-dependent Hartree-Fock calculations suggests strong pairing correlations between the two protons. Excitation energies in the projectile-like fragments up to ~ 15MeV for the 16O and ~ 25MeV for 32S-induced reactions demonstrate the population of highly excited states in the residual nuclei, indicating substantial dissipation of kinetic energy. These highly inelastic (large excitation energies) and complex (correlated few-nucleon transfer) processes may be closely related to the depletion of fusion through tunnelling at sub-barrier energies.