diff --git a/lib/pagelib.php b/lib/pagelib.php
index 7e173071f40..99366fc3872 100644
--- a/lib/pagelib.php
+++ b/lib/pagelib.php
@@ -1572,6 +1572,15 @@ class moodle_page {
         $this->_wherethemewasinitialised = debug_backtrace();
     }
 
+    /**
+     * For diagnostic/debugging purposes, find where the theme setup was triggered.
+     *
+     * @return null|array null if theme not yet setup. Stacktrace if it was.
+     */
+    public function get_where_theme_was_initialised() {
+        return $this->_wherethemewasinitialised;
+    }
+
     /**
      * Reset the theme and output for a new context. This only makes sense from
      * external::validate_context(). Do not cheat.
diff --git a/lib/testing/generator/module_generator.php b/lib/testing/generator/module_generator.php
index 5400648bc56..4a2396e552a 100644
--- a/lib/testing/generator/module_generator.php
+++ b/lib/testing/generator/module_generator.php
@@ -222,11 +222,15 @@ abstract class testing_module_generator extends component_generator_base {
      *     cmid (corresponding id in course_modules table)
      */
     public function create_instance($record = null, array $options = null) {
-        global $CFG, $DB;
+        global $CFG, $DB, $PAGE;
         require_once($CFG->dirroot.'/course/modlib.php');
 
         $this->instancecount++;
 
+        // Creating an activity is a back end operation, which should not cause any output to happen.
+        // This will allow us to check that the theme was not initialised while creating the module instance.
+        $outputstartedbefore = $PAGE->get_where_theme_was_initialised();
+
         // Merge options into record and add default values.
         $record = $this->prepare_moduleinfo_record($record, $options);
 
@@ -269,6 +273,19 @@ abstract class testing_module_generator extends component_generator_base {
         // Prepare object to return with additional field cmid.
         $instance = $DB->get_record($this->get_modulename(), array('id' => $moduleinfo->instance), '*', MUST_EXIST);
         $instance->cmid = $moduleinfo->coursemodule;
+
+        // If the theme was initialised while creating the module instance, something somewhere called an output
+        // function. Rather than leaving this as a hard-to-debug situation, let's make it fail with a clear error.
+        $outputstartedafter = $PAGE->get_where_theme_was_initialised();
+
+        if ($outputstartedbefore === null && $outputstartedafter !== null) {
+            throw new coding_exception('Creating a mod_' . $this->get_modulename() . ' activity initialised the theme and output!',
+                'This should not happen. Creating an activity should be a pure back-end operation. Unnecessarily initialising ' .
+                'the output mechanism at the wrong time can cause subtle bugs and is a significant performance hit. There is ' .
+                'likely a call to an output function that caused it:' . PHP_EOL . PHP_EOL .
+                format_backtrace($outputstartedafter, true));
+        }
+
         return $instance;
     }