Only time it's happened to me recently was LH coach going to South Africa, amazingly the check-in agent at LHR even asked me if I wanted a window or aisle, and told me she'd sorted me aisles on both ways. I checked the BPs and saw both seats were B but decided it must be a 2-5-2 config or something (next time I won't give them benefit of the doubt). Anyway, she'd kindly given me middles in both segments and the LH gate-staff where absolutely unwilling to do anything about it.