- First take at improving the user module's code: replaced the custom error
  printing with calls to drupal_set_message().

modules/user.module

@@ -648,7 +648,7 @@ function user_login($edit = array(), $msg = "") {
   */
 
   if ($error) {
-    $output .= theme("error", $error);
+    drupal_set_message($error, 'error');
   }
 
   /*
@@ -753,7 +753,7 @@ function user_pass($edit = array()) {
 
     // Display error message if necessary.
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
 
     /*
@@ -876,7 +876,7 @@ function user_register($edit = array()) {
   }
   else {
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
   }
 
@@ -981,7 +981,7 @@ function user_edit($edit = array()) {
     }
 
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
 
     if (!$edit) {
@@ -1172,7 +1172,7 @@ function user_admin_create($edit = array()) {
   else {
 
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
 
     $output .= form_textfield(t("Username"), "name", $edit["name"], 30, 55, t("Provide the username of the new account."));
@@ -1429,7 +1429,7 @@ function user_admin_edit($edit = array()) {
         drupal_set_message(t("user information changes have been saved."));
       }
       else {
-        $output .= theme("error", $error);
+        drupal_set_message($error, 'error');
       }
     }
     else if ($op == t("Delete account")) {
@@ -1441,7 +1441,7 @@ function user_admin_edit($edit = array()) {
       }
       else {
         $error = t("Failed to delete account: the account has to be blocked first.");
-        $output .= theme("error", $error);
+        drupal_set_message($error, 'error');
       }
     }
 

modules/user/user.module

@@ -648,7 +648,7 @@ function user_login($edit = array(), $msg = "") {
   */
 
   if ($error) {
-    $output .= theme("error", $error);
+    drupal_set_message($error, 'error');
   }
 
   /*
@@ -753,7 +753,7 @@ function user_pass($edit = array()) {
 
     // Display error message if necessary.
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
 
     /*
@@ -876,7 +876,7 @@ function user_register($edit = array()) {
   }
   else {
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
   }
 
@@ -981,7 +981,7 @@ function user_edit($edit = array()) {
     }
 
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
 
     if (!$edit) {
@@ -1172,7 +1172,7 @@ function user_admin_create($edit = array()) {
   else {
 
     if ($error) {
-      $output .= theme("error", $error);
+      drupal_set_message($error, 'error');
     }
 
     $output .= form_textfield(t("Username"), "name", $edit["name"], 30, 55, t("Provide the username of the new account."));
@@ -1429,7 +1429,7 @@ function user_admin_edit($edit = array()) {
         drupal_set_message(t("user information changes have been saved."));
       }
       else {
-        $output .= theme("error", $error);
+        drupal_set_message($error, 'error');
       }
     }
     else if ($op == t("Delete account")) {
@@ -1441,7 +1441,7 @@ function user_admin_edit($edit = array()) {
       }
       else {
         $error = t("Failed to delete account: the account has to be blocked first.");
-        $output .= theme("error", $error);
+        drupal_set_message($error, 'error');
       }
     }