=== modified file 'lib/lp/testing/layers.py'
--- lib/lp/testing/layers.py	2015-12-16 11:48:10 +0000
+++ lib/lp/testing/layers.py	2015-12-16 17:12:50 +0000
@@ -57,6 +57,7 @@ import errno
 import gc
 import logging
 import os
+import pdb
 import signal
 import socket
 import subprocess
@@ -468,6 +469,7 @@ class BaseLayer:
             del gc.garbage[:]
             gc.collect()  # Expensive, so only do if there might be garbage.
             if gc.garbage:
+                pdb.set_trace()
                 BaseLayer.flagTestIsolationFailure(
                         "Test left uncollectable garbage\n"
                         "%s (referenced from %s; referencing %s)"