Handling error states with admin_post

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You can use set_transient with the user_id if you are relying on users being logged in, otherwise, cookie would probably be better. For logged in users, you can have the user_id get set within the name of the transient. An example here:

function form_post() {
    $user_info = wp_get_current_user();
    $user_id = $user_info->exists() && !empty($user_info->ID) ? $user_info->ID : 0;
    $url = wp_get_referer();

    // Validate the form fields and populate this array when errors occur
    $validation_errors = array();  // I like to use the id of the elements as keys and the error string as the values of the array, so i can add error class to the elements if needed easily...

    // Now to save the transient...
    if (!empty($validation_errors)) {
        set_transient('validation_errors_' . $user_id, $my_errors);
        wp_redirect($url);
        die();
    }
}

Now before you output your form element on your page, you just need to check the transient for that user, if exists, output the errors, than delete the transient right after that…

For example:

<?php 
$user_info = wp_get_current_user();

$user_id = $user_info->exists() && !empty($user_info->ID) ? $user_info->ID : 0;
$validation_errors = get_transient('validation_errors_' . $user_id);

if ($validation_errors !== FALSE) {
   echo '<div class="errors">Please fix the following Validation Errors:<ul><li>' . implode('</li><li>', $validation_errors) . '</li></ul></div>';
   delete_transient('validation_errors_' . $user_id);
} ?>

<form ...>

</form>

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