Quick-n-dirty patch to Drupal 7 bootstrap to examine backtraces on error messages. Requires devel enabled.

backtrace-errors-bootstrap.inc.patch

diff --git a/includes/bootstrap.inc b/includes/bootstrap.inc
index fe3d7a7..c2c95cc 100644
--- a/includes/bootstrap.inc
+++ b/includes/bootstrap.inc
@@ -1794,6 +1794,11 @@ function watchdog($type, $message, $variables = array(), $severity = WATCHDOG_NO
  */
 function drupal_set_message($message = NULL, $type = 'status', $repeat = TRUE) {
   if ($message) {
+
+    if ($type == 'error') {
+      $message = kprint_r( debug_backtrace(), TRUE, $message );
+    }
+
     if (!isset($_SESSION['messages'][$type])) {
       $_SESSION['messages'][$type] = array();
     }