The middle of England

The mistake

The store locator geocodes the customer's address and finds the closest shop to those coordinates, but when geocoding fails it initialises the search origin to a hardcoded default point instead of returning an error. That default is the geographic centre of the country, so every unresolvable address falls back to the same coordinates and returns the same nearest store. Swapping the default for the office's coordinates doesn't fix the bug, it just moves the crowd.