During plugin activation, WP 3.0 checks …

During pluginPlugin A plugin is a piece of software containing a group of functions that can be added to a WordPress website. They can extend functionality or add new features to your WordPress websites. WordPress plugins are written in the PHP programming language and integrate seamlessly with WordPress. These can be free in the WordPress.org Plugin Directory https://wordpress.org/plugins/ or can be cost-based plugin from a third-party 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 tagtag A directory in Subversion. WordPress uses tags to store a single snapshot of a version (3.6, 3.6.1, etc.), the common convention of tags in version control systems. (Not to be confused with post tags.) 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.

See tickets 13585 and 12089.

#3-0, #dev-notes