If you were looking for vulnerabilities on a website, you might open up the original page source looking for commented-out code, javascript source, hidden forms, etc. If you suspected an XSS attack on your own site, chances are you might right-click on the page and view source to check for unwanted scripts. If you needed to register for CTP, hack this site, or read the snarky comments in the HTML of www.defcon.org, you would probably need to view the page source.

This is all based on your assumption that when you right-click on the page and select “View Source” the text you see is the HTML source that the server sent to your browser when it requested the URL in your address bar. Unfortunately if you assumed this, you would be wrong […]