diff --git a/lib/filelib.php b/lib/filelib.php
index 3dcebbf5f74..c8e98f503dd 100644
--- a/lib/filelib.php
+++ b/lib/filelib.php
@@ -2219,20 +2219,20 @@ function readfile_accel($file, $mimetype, $accelerate) {
             if ($ranges) {
                 if (is_object($file)) {
                     $handle = $file->get_content_file_handle();
+                    if ($handle === false) {
+                        throw new file_exception('storedfilecannotreadfile', $file->get_filename());
+                    }
                 } else {
                     $handle = fopen($file, 'rb');
+                    if ($handle === false) {
+                        throw new file_exception('cannotopenfile', $file);
+                    }
                 }
                 byteserving_send_file($handle, $mimetype, $ranges, $filesize);
             }
         }
     }
 
-    header('Content-Length: '.$filesize);
-
-    if (!empty($_SERVER['REQUEST_METHOD']) and $_SERVER['REQUEST_METHOD'] === 'HEAD') {
-        exit;
-    }
-
     if ($filesize > 10000000) {
         // for large files try to flush and close all buffers to conserve memory
         while(@ob_get_level()) {
@@ -2242,11 +2242,21 @@ function readfile_accel($file, $mimetype, $accelerate) {
         }
     }
 
+    // Send this header after we have flushed the buffers so that if we fail
+    // later can remove this because it wasn't sent.
+    header('Content-Length: ' . $filesize);
+
+    if (!empty($_SERVER['REQUEST_METHOD']) and $_SERVER['REQUEST_METHOD'] === 'HEAD') {
+        exit;
+    }
+
     // send the whole file content
     if (is_object($file)) {
         $file->readfile();
     } else {
-        readfile_allow_large($file, $filesize);
+        if (readfile_allow_large($file, $filesize) === false) {
+            throw new file_exception('cannotopenfile', $file);
+        }
     }
 }
 
diff --git a/lib/filestorage/file_system.php b/lib/filestorage/file_system.php
index 8cf9ffcdc6d..50156960d78 100644
--- a/lib/filestorage/file_system.php
+++ b/lib/filestorage/file_system.php
@@ -63,7 +63,9 @@ abstract class file_system {
         } else {
             $path = $this->get_remote_path_from_storedfile($file);
         }
-        readfile_allow_large($path, $file->get_filesize());
+        if (readfile_allow_large($path, $file->get_filesize()) === false) {
+            throw new file_exception('storedfilecannotreadfile', $file->get_filename());
+        }
     }
 
     /**
diff --git a/lib/setuplib.php b/lib/setuplib.php
index f291a6dfc1f..4aa3b9ad136 100644
--- a/lib/setuplib.php
+++ b/lib/setuplib.php
@@ -362,6 +362,10 @@ function default_exception_handler($ex) {
 
     $info = get_exception_info($ex);
 
+    // If we already tried to send the header remove it, the content length
+    // should be either empty or the length of the error page.
+    @header_remove('Content-Length');
+
     if (is_early_init($info->backtrace)) {
         echo bootstrap_renderer::early_error($info->message, $info->moreinfourl, $info->link, $info->backtrace, $info->debuginfo, $info->errorcode);
     } else {