Following along with the official guide from Facebook about creating a simple application for the site, Dave Courtney ran into some issues. Specifically, a fatal error when he tried to run his three line script.

After setting up my application settings, which were all straight forward, I set about creating a simple 'Hello World' page. Following the step by step guide to creating an application provided by FaceBook I wrote a simple index.php page. [...] This is where I encountered probably the only major issue of the day.

In the end, his issue was caused by the SimpleXML he had installed locally. Once his application was uploaded, it worked just fine. With this problem down, he moved on to working with the Facebook Markup Language and how he integrated it with the database.