How to delete all cookies in safari 5.1.3 on osx lion
Safari


  Privacy Pane 
  A new Privacy pane in Safari preferences gives you more
  control over your online privacy, allowing you to clear website data,
  customize cookie settings, and choose whether websites can request
  your location information.


This is also a known issue in some versions of Safari when working with the YouTube HTML5 Beta. 

Since it is not a Safari or OS X issue, you may want to try other browsers like  FireFox or Chrome.

Lastly, Always make sure OS X is updated along with Safari. Safari is currently at 5.1.5 right now. Here is how to update