50-100 years ago, many major cities connected the storm sewer system to the sanitary sewer system to save money. It's a good idea as long as it doesn't rain too hard.
About 30 years ago, while living in Milwaukee, there was a super-heavy rainstorm. I recall seeing water 8-10" high running down the residential street behind the building where I worked. News pictures on TV that night showed the old Milwaukee County Stadium with more than 3 feet of water on the infield!
One of my friends had a geyser in his basement through the floor drain. Being 'lower' in town (4 blocks from Lake Michigan in Bay View) than most of the rest of Milwaukee all the storm water tried to run to the Jones Island processing plant. Needless to say, everything in the basement had to be discarded. All interior partitions had to be replaced and walls bleached to remove the untreated sewage odor.
As is (perhaps, still) customary during heavy rainfalls, they 'open the gates' and let everything go straight out to Lake Michigan without processing it. They can't handle the volume. Milwaukee and Chicago have/are digging 'deep tunnels' as a means to hold the massive runoffs until they can be pumped at a slower volume for processing when the weather is dry. I don't know how many other cities have built deep tunnels to hold the rainwater, but where I am in New England today, the same thing still holds true...except they have a large river to dump into.