A common error occurs when including a URL which contains an ampersand (“&”). WordPress Twitter Tools Plugin by Alex King do this, I mean put & in the url. So we have to replace & with it’s equivalent &
Follow the procedure :
1. Always Backup the original File
2. Find the twitter-tools.php file in Twitter Tools Plugin folder
3. Find the following piece of code in the file
Replace the Code with this
Go to XHTML Validation Service and re-validate.