At WordCamp Montréal 2013, Yannick Lefebvre talked about WordPress plugins from a user/integrator perspective. Slides. Here are my notes:
Things to check before using a plugin
Compatability - does it work for your version?
# of Downloads - higher counts are generally a good indicator.
Ratings - take with a grain of salt, because anyone can vote.
# of Resolved Threads - doesn’t take into account support delivered outside of WordPress.org. Some developers use their own forums for support, and are very active there.
Things to do before deploying a 3rd-party plugin
Check render time before installing it, and then check after. Check both the front and back end. Are the load times acceptable? Lucas’ note: if your plugin provides a feature only used on select pages (e.g. a shortcode), make sure to check the performance of those pages especially.
Log out and view the site as a regular user. Does it still work?
Troubleshooting a faulty plugin
Deactivate the last plugin installed.
Deactivate all plugins and activate each one again, one at a time. Refresh your site each time, see when it breaks.
Read the plugin page, check for install instructions you may have missed.
Contact the author if all else fails and you absolutely need the plugin to work.
Conflicting styles between the plugin’s styles and yours, or one plugin’s styles and another.
Use the inspector to find the “winning styles.”
Can modify the theme to fix these issues. !important can come in handy here.
Can modify the plugin, but make sure to backup the files first and know that your changes will be overwritten if you automatically update.
Often breaks all the jQuery-affected elements on a page.
Check the console of the inspector for errors.
Common issue is more than one jQuery verison being loaded. Check for multiple copies in the source, or in the inspector’s scripts section.
When using jQuery, use the longform version (jQuery()) instead of the shorthand ($()) to avoid conflicts.
Write permissions may be wrong on the upload folder. Lucas’ note: 755 should do the trick, according to the WordPress Codex.
The server max upload size (set in php.ini) may be too small. By default it’s 8MB, but some hosts turn it down. Contact yours to see how to change it.
Troubleshooting bad upgrades
Reactivate the plugin. (Especially when uploaded with FTP).
Check for error messages, and read them.
Revert to a previous version (you do have a backup, don’t you?).
Find previous version (if you don’t have a backup), by browsing the Trac page for the plugin (when on WordPress.org) and checking for tags, or showing revisions. Lucas’ note: just make a backup regularly, it’ll save you so much hassle in the long run.
Bad plugin breakups
You may “break up” with a plugin when you find one that’s a better solution for your needs, or if what you’re using now provides unnecessary functionality.
The plugin development may have ended, and it may not support new versions of WordPress.
Make sure to analyse the effects of deactivation. Plugin authors rarely remember to include features which clean up after being uninstalled, so you may have to do this yourself.