:( [I][/If there is a main sewage pump in an apartment complex not working properly, is it possible for most toilets in a certain building connected to that pump to not flush, and one or more to overflow drastically because of the pressure pushing water from the other units to one or more other units that do overflow?

I]

Sure it's possible -- If the ejection basin is an airtight system.

This being an apartment complex, why isn't there a backup pump on the system?