Even the biggest sites have issues sometimes. 

An undisclosed "DNS-related issue" late Saturday knocked several Google services offline, prompting widespread speculation that the Web search giant fell victim to the recent wave of DNS cache-poisoning attacks. The outage lasted for several hours and affected the Google.com home page, Gmail, Google News, Froogle, Google Images, Google Groups and Google Local. The outage also caused service failure on advertisements from Google’s AdSense service.

Read full story: Google Knocked Offline by ‘DNS Issue’