During plugin activation, WP 3.0 checks to see if the plugin generates any output. If any output is generated a warning is shown to the user. Generating output during activation is usually a good sign that the plugin has spaces before the opening php tag or that it generates warnings. This extra output can break feeds and cause “headers already sent” warnings. Please test your plugins against 3.0 and make sure your they do not generate unexpected output.
gaurab, and 1 other are discussing. 
mike503 4:38 am on June 24, 2010 Permalink
The whitespace issue can usually be alleviated by not ending your script with ?>
It seems wrong, but Zend Framework, Drupal and many other projects have adopted it as a standard, and nobody actually has a problem with it from a technical angle from the PHP side when I’ve ever asked.
gaurab 8:31 am on June 27, 2010 Permalink
I am having the same problem during activation.When i try to activate the plugin i get the message “The plugin generated 560 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.”
So where can i edit the plugin to get rid of this.It works in all of the lower wp-versions.
Jeff Lambert 8:01 pm on October 16, 2010 Permalink
If you are still having an issue, I found a spot in a plugin I was developing, which may be the same issue with this plugin. Check that the “add_option” method does not contain a “description” parameter of, if it does, that it is an empty string or null. “Description” was deprecated with version 2.3 of WordPress. Good luck.
manie 6:53 am on October 31, 2011 Permalink
Hey guys I’m having the same issue. However, I’m getting the following message:
“The plugin generated 3 characters of unexpected output during activation. If you notice “headers already sent” messages, problems with syndication feeds or other issues, try deactivating or removing this plugin.”
I’m crap at editing code, was this what you was hinting at Jeff Lamberth?
Please advise.