I'm not talking about combined sewers. Systems in which the sanitary sewer and the storm sewer are completely separate can become combined sewers when the sanitary sewer is inundated with more infiltration/inflow than it can handle. When this happens, the sanitary sewer overflows into the street or people's homes. That sewage overflowing into the street will then make its way into the storm sewer.

There is no easy solution to this. Building the sanitary sewers bigger is the obvious solution, except that costs more money and a lot of places don't have much money to spare (corruption, poverty, etc.) Furthermore, just how over-designed does a sanitary sewer have to be? Climate change makes this difficult to predict decades in advance.

BTW [intentionally] combined sewers are problematic even when they're not overflowing. Diluting sewage with a bunch of water increases the cost and decreases the efficacy of treatment.