The article Fixing extensions without modifying their code reminded me of another patching technique that I used. My conviction and a hard look on the web had led me to an article Redefine PHP class methods or class giving me a head start to do what I wanted: patching third party code without touching the original to allow easy upgrades.
If you're working a lot with extensions you often stumble upon problems, when you want to include them into your custom web application, like hardcoded or absolute path aliases or classes extended from core application components, which implement addiditonal features.
I recently had to have a checkbox column in my grid view that supported the 'disabled' attribute. This was easy enough to do by extending the CCheckBoxColumn. However, I also wanted the checkbox to be selected when clicking on the table's row (supported via CGridView's selectableRows option). I discovered I had to tweak yiigridview.js so that it would not select disabled rows when clicking on it or on the 'select all' checkbox.
In many cases developers of one project use their own local environments for coding. There may be a common test platform and, of course, production. All these environments usually have different DB credentials and directory structures (first of all, path to YII framework directory). Another feature is debugging. It's convenient to turn it on for local environments, eventually - on the test server and never (generally speaking) - on the production.