diff --git a/lib/outputrequirementslib.php b/lib/outputrequirementslib.php
index 0e55357bcb1..18aea452179 100644
--- a/lib/outputrequirementslib.php
+++ b/lib/outputrequirementslib.php
@@ -195,6 +195,15 @@ class page_requirements_manager {
         $this->YUI_config->comboBase    = $this->yui3loader->comboBase;
         $this->YUI_config->combine      = $this->yui3loader->combine;
 
+        // If we've had to patch any YUI modules between releases, we must override the YUI configuration to include them.
+        // For important information on patching YUI modules, please see http://docs.moodle.org/dev/YUI/Patching.
+        if (!empty($CFG->yuipatchedmodules) && !empty($CFG->yuipatchlevel)) {
+            $this->YUI_config->define_patched_core_modules($this->yui3loader->local_comboBase,
+                    $CFG->yui3version,
+                    $CFG->yuipatchlevel,
+                    $CFG->yuipatchedmodules);
+        }
+
         $configname = $this->YUI_config->set_config_source('lib/yui/config/yui2.js');
         $this->YUI_config->add_group('yui2', array(
             // Loader configuration for our 2in3, for now ignores $CFG->useexternalyui.
@@ -1773,6 +1782,59 @@ class YUI_config {
         }
         return $modules;
     }
+
+    /**
+     * Define YUI modules which we have been required to patch between releases.
+     *
+     * We must do this because we aggressively cache content on the browser, and we must also override use of the
+     * external CDN which will serve the true authoritative copy of the code without our patches.
+     *
+     * @param String combobase The local combobase
+     * @param String yuiversion The current YUI version
+     * @param Int patchlevel The patch level we're working to for YUI
+     * @param Array patchedmodules An array containing the names of the patched modules
+     * @return void
+     */
+    public function define_patched_core_modules($combobase, $yuiversion, $patchlevel, $patchedmodules) {
+        // The version we use is suffixed with a patchlevel so that we can get additional revisions between YUI releases.
+        $subversion = $yuiversion . '_' . $patchlevel;
+
+        if ($this->comboBase == $combobase) {
+            // If we are using the local combobase in the loader, we can add a group and still make use of the combo
+            // loader. We just need to specify a different root which includes a slightly different YUI version number
+            // to include our patchlevel.
+            $patterns = array();
+            $modules = array();
+            foreach ($patchedmodules as $modulename) {
+                // We must define the pattern and module here so that the loader uses our group configuration instead of
+                // the standard module definition. We may lose some metadata provided by upstream but this will be
+                // loaded when the module is loaded anyway.
+                $patterns[$modulename] = array(
+                    'group' => 'yui-patched',
+                );
+                $modules[$modulename] = array();
+            }
+
+            // Actually add the patch group here.
+            $this->add_group('yui-patched', array(
+                'combine' => true,
+                'root' => $subversion . '/',
+                'patterns' => $patterns,
+                'modules' => $modules,
+            ));
+
+        } else {
+            // The CDN is in use - we need to instead use the local combobase for this module and override the modules
+            // definition. We cannot use the local base - we must use the combobase because we cannot invalidate the
+            // local base in browser caches.
+            $fullpathbase = $combobase . $subversion . '/';
+            foreach ($patchedmodules as $modulename) {
+                $this->modules[$modulename] = array(
+                    'fullpath' => $fullpathbase . $modulename . '/' . $modulename . '-min.js'
+                );
+            }
+        }
+    }
 }
 
 /**
diff --git a/lib/setup.php b/lib/setup.php
index 94115d43b2e..1618676f441 100644
--- a/lib/setup.php
+++ b/lib/setup.php
@@ -349,6 +349,15 @@ if (!defined('AJAX_SCRIPT')) {
 $CFG->yui2version = '2.9.0';
 $CFG->yui3version = '3.13.0';
 
+// Patching the upstream YUI release.
+// For important information on patching YUI modules, please see http://docs.moodle.org/dev/YUI/Patching.
+// If we need to patch a YUI modules between official YUI releases, the yuipatchlevel will need to be manually
+// incremented here. The module will also need to be listed in the yuipatchedmodules.
+// When upgrading to a subsequent version of YUI, these should be reset back to 0 and an empty array.
+$CFG->yuipatchlevel = 0;
+$CFG->yuipatchedmodules = array(
+);
+
 // Store settings from config.php in array in $CFG - we can use it later to detect problems and overrides.
 if (!isset($CFG->config_php_settings)) {
     $CFG->config_php_settings = (array)$CFG;
diff --git a/lib/yuilib/readme_moodle.txt b/lib/yuilib/readme_moodle.txt
index 7c7a058eae0..64da716e370 100644
--- a/lib/yuilib/readme_moodle.txt
+++ b/lib/yuilib/readme_moodle.txt
@@ -12,6 +12,9 @@ Description of import of various YUI libraries into Moodle:
 * update lib/thrirdpartylibs.xml
 * verify our simpleyui rollup contents in /theme/yui_combo.php
 
+If you need to patch the YUI library between its official releases, you *must* read
+http://docs.moodle.org/dev/YUI/Patching.
+
 3/ YUI3 Gallery version gallery-2013.10.02-20-26:
 * selective copy of the "build" directory for the checked out tag of yui3-gallery.
   Unit test code coverage files (*-coverage.js) are removed but no other changes are made.
diff --git a/theme/yui_combo.php b/theme/yui_combo.php
index c19a9f13be6..83221392363 100644
--- a/theme/yui_combo.php
+++ b/theme/yui_combo.php
@@ -102,6 +102,11 @@ while (count($parts)) {
                 continue;
             }
 
+            // Allow support for revisions on YUI between official releases.
+            // We can just discard the subrevision since it is only used to invalidate the browser cache.
+            $yuipatchedversion = explode('_', $revision);
+            $yuiversion = $yuipatchedversion[0];
+
             $yuimodules = array(
                 // Include everything from original SimpleYUI,
                 // this list can be built using http://yuilibrary.com/yui/configurator/ by selecting all modules
@@ -193,14 +198,14 @@ while (count($parts)) {
             if ($type === 'js') {
                 $newparts = array();
                 foreach ($yuimodules as $module) {
-                    $newparts[] = $revision . '/' . $module . '/' . $module . $filesuffix;
+                    $newparts[] = $yuiversion . '/' . $module . '/' . $module . $filesuffix;
                 }
                 $newparts[] = 'yuiuseall/yuiuseall';
                 $parts = array_merge($newparts, $parts);
             } else {
                 $newparts = array();
                 foreach ($yuimodules as $module) {
-                    $candidate =  $revision . '/' . $module . '/assets/skins/sam/' . $module . '.css';
+                    $candidate =  $yuiversion . '/' . $module . '/assets/skins/sam/' . $module . '.css';
                     if (!file_exists("$CFG->libdir/yuilib/$candidate")) {
                         continue;
                     }
@@ -306,10 +311,17 @@ while (count($parts)) {
         $filecontent = "var Y = YUI().use('*');";
 
     } else {
-        if ($version != $CFG->yui3version) {
+        // Allow support for revisions on YUI between official releases.
+        // We can just discard the subrevision since it is only used to invalidate the browser cache.
+        $yuipatchedversion = explode('_', $version);
+        $yuiversion = $yuipatchedversion[0];
+        if ($yuiversion != $CFG->yui3version) {
             $content .= "\n// Wrong yui version $part!\n";
             continue;
         }
+        $newpart = explode('/', $part);
+        $newpart[0] = $yuiversion;
+        $part = implode('/', $newpart);
         $contentfile = "$CFG->libdir/yuilib/$part";
     }
     if (!file_exists($contentfile) or !is_file($contentfile)) {
diff --git a/theme/yui_image.php b/theme/yui_image.php
index 5a8818eb48d..5865aa37f7a 100644
--- a/theme/yui_image.php
+++ b/theme/yui_image.php
@@ -63,15 +63,21 @@ if ($version == 'moodle' && count($parts) >= 3) {
 } else if ($version == 'gallery' && count($parts) >= 3) {
     list($revision, $module, , , , $image) = $parts;
     $imagepath = "$CFG->dirroot/lib/yuilib/gallery/$module/assets/skins/sam/$image";
-} else if (count($parts) == 1 && ($version == $CFG->yui3version || $version == $CFG->yui2version)) {
-    list($image) = $parts;
-    if ($version == $CFG->yui3version) {
-        $imagepath = "$CFG->dirroot/lib/yuilib/$CFG->yui3version/assets/skins/sam/$image";
-    } else  {
-        $imagepath = "$CFG->dirroot/lib/yuilib/2in3/$CFG->yui2version/build/assets/skins/sam/$image";
-    }
 } else {
-    yui_image_not_found();
+    // Allow support for revisions on YUI between official releases.
+    // We can just discard the subrevision since it is only used to invalidate the browser cache.
+    $yuipatchedversion = explode('_', $version);
+    $yuiversion = $yuipatchedversion[0];
+    if (count($parts) == 1 && ($yuiversion == $CFG->yui3version || $yuiversion == $CFG->yui2version)) {
+        list($image) = $parts;
+        if ($yuiversion == $CFG->yui3version) {
+            $imagepath = "$CFG->dirroot/lib/yuilib/$CFG->yui3version/assets/skins/sam/$image";
+        } else  {
+            $imagepath = "$CFG->dirroot/lib/yuilib/2in3/$CFG->yui2version/build/assets/skins/sam/$image";
+        }
+    } else {
+        yui_image_not_found();
+    }
 }
 
 if (!file_exists($imagepath)) {