A badly built site that needed stabilising before anything could be added to it.
Got it stable first, then started building on it properly. The largest addition since was an online inventory system that lets a customer choose their next piece from a collection.
Far fewer support messages about things not working, which gave them their time back to spend on the business rather than on the website. With me since early 2024.
I audited it before touching anything. Two of the things that came back: any logged-in member could open somebody else's booking calendar by guessing the URL and place a delivery booking on their behalf, and the emails reminding members to book their next delivery were fetching "the current user" from a scheduled task, which has no current user. Those emails had never worked and never could have.