Doesn't matters if it's a new system which needs a solution for a problem, or if it's an existing system having a bug which needs to be catched: I have experience with both of these situations.
Whatever other people say, architecture is the most important factor of a web application. I'm very concerned about building things the right way, separating the layers, making the code understandable and modular. I always prefer a well thought architecture instead of "fastly wired" code.
I like to think that every project has a good chance to grow big. This means that I'm chosing the right tools from the beginning in order to make sure that you'll be able to easily scale your project.