Tag
Finding the actual cause when WordPress breaks.
WordPress Tutorials
admin-ajax.php picks one of two hooks based on whether the visitor is logged in, never both, and answers 400 with a bare 0 when nothing is listening. Most broken AJAX handlers fail on one of those two facts.
WooCommerce
WooCommerce blocks the order with this message when the cart needs shipping but it has no country to ship to, or when shipping costs are hidden until an address is complete and the address never becomes complete.
WooCommerce
The notice is not about broken product data. It means the archive query for this exact URL returned zero rows, and the cause is usually visible in the URL, the visibility terms or a stale lookup table.
WooCommerce
WooCommerce says a payment method is invalid when the gateway the shopper picked is no longer in the list of available gateways by the time the order is submitted. Here is how to find out what removed it.
WooCommerce
WooCommerce shows the expired-session notice when the checkout arrives with an empty cart, usually because the session cookie never reached the server or a cached page carried someone else's nonce.
Troubleshooting
The REST server ran and found no registered route that matches both the path and the HTTP method. Either the route was never registered for this request, the URL does not match its pattern, or the method is wrong — and the message is the same for all three.
Troubleshooting
The script ran before jQuery existed on the page, or on a page where nothing asked WordPress to load jQuery at all. The fix is to declare the dependency, not to add another copy of the library.
Troubleshooting
A redirect loop is two rules that disagree about the correct URL, each sending the browser to the other's answer. Trace the hops with curl, and the two Location headers tell you which layers are fighting: proxy, server, WordPress or a plugin.
Troubleshooting
The log line names the missing table and the code that asked for it. A core table, a plugin's table and a table with the wrong prefix are three different problems, and only one of them is fixed by reactivating a plugin.
Troubleshooting
When the error says the arguments were passed in class-wp-hook.php, WordPress called your function with fewer values than it declares. The cause is nearly always the fourth parameter of add_action or add_filter, which defaults to one.
Troubleshooting
PHP 8.2 started warning whenever code assigns a property a class never declared. The site keeps working. The notice tells you which plugin or theme has not been updated for modern PHP, and you see it only because debugging is switched on.
Troubleshooting
PHP refused to compile a file, so nothing in it ran. The message names the file and a line, but the line is where PHP gave up, which is often one line after the actual mistake. Here is how to read it and get the site back without the dashboard.