g_warning() is a printf-like function and its first argument should
be a format string.

--- src/mainwindow.c
+++ src/mainwindow.c
@@ -634,7 +634,7 @@
   /* create the open file thread */
   if(g_thread_create(open_torrent_file, filename, FALSE, &err) == NULL)
   {
-    g_warning(err->message);
+    g_warning("%s", err->message);
     g_free(filename);
     g_error_free(err);
   } 
@@ -671,7 +671,7 @@
 
   if(g_thread_create(tracker_scrape, tracker, FALSE, &err) == NULL)
   {
-    g_warning(err->message);
+    g_warning("%s", err->message);
     g_free(tracker);
     g_error_free(err);
   } 
@@ -722,7 +722,7 @@
        filename = gtk_file_chooser_get_filename(GTK_FILE_CHOOSER(dialog));
        if(g_thread_create(check_files, filename, FALSE, &err) == NULL)
        {
-         g_warning(err->message);
+         g_warning("%s", err->message);
          g_free(filename); 
          g_error_free(err);
        }
@@ -755,7 +755,7 @@
 
   if(g_thread_create(tracker_scrape, tracker, FALSE, &err) == NULL)
   {
-    g_warning(err->message);
+    g_warning("%s", err->message);
     g_error_free(err);
   } 
   
@@ -853,7 +853,7 @@
     {
       if(g_thread_create(open_torrent_file, filename, FALSE, &err) == NULL)
       {
-        g_warning(err->message);
+        g_warning("%s", err->message);
         g_free(filename);
         g_error_free(err);
       }