I used post meta (in a unusual way) instead of transients as regenerating the data was too expensive. And I learned about fields=’ids’ in WP_Query.
Use the $wp_filter global variable to see the functions attached to an action. And see how *not* to use it!
Allow customers with a company view orders of other customers with the same company. Based on a query on boards.ie.
A shortcode to display the number of years, weeks or days since the specified date. Keeps sentences like “working with WordPress for X years” current.
Use Advanced Custom Fields PRO Repeater field in an options page to make a list of information easy for a client to maintain for easy rendering in multiple formats.
Learn how to disable plugins except for specified urls. Inspired by Plugin Organizer plugin, I disabled CF7 for all but the contact page.