Index: includes/theme.inc
===================================================================
--- includes/theme.inc	(revision 10424)
+++ includes/theme.inc	(working copy)
@@ -32,6 +32,12 @@
 function init_theme() {
   global $theme, $user, $custom_theme, $theme_engine, $theme_key;
 
+  $backtrace=debug_backtrace();
+  if(arg(0)=='admin' && arg(1)=='build' && arg(2)=='block' && $backtrace[sizeof($backtrace)-1]['function']=='drupal_bootstrap'){  	  	
+  	die("Error: Attempt to initialise theme in bootstrap phase. This is usually caused by a call to path_to_theme() within a hook_menu or a module file.\n<xmp>".
+  		print_r($backtrace,true)."</xmp>");
+  }
+
   // If $theme is already set, assume the others are set, too, and do nothing
   if (isset($theme)) {
     return;