Headless browsers like Zombie.js are useful for integration testing Node.js web applications due to their speed and asynchronous nature. However, they may not always behave the same way as "real" browsers, leading to issues when testing third-party OAuth login flows. To overcome these challenges, developers can create test users, pre-approve their app in a real browser, peek at the page using browser.html(), change the user agent to mimic a modern desktop browser, and get creative with workarounds for specific issues. A GitHub repository demonstrates examples of logging into an application with Google and LinkedIn, which may be useful even when testing other social services.