The select_city select in the patron editor ( code in member-main-address-style-us.inc ) will select the wrong option of there are multiple options with the same city but different zipcodes. For example, let's say a city has 3 zip codes 16001, 16002, and 16003. If I live in 16003, the chooser will show and have selected the 16001 zip code. This is because the "selected" modifier is set for each option if just the city is a match, ignoring the zipcode, state and country.
*** This bug has been marked as a duplicate of bug 15644 ***