diff --git a/djblets/extensions/manager.py b/djblets/extensions/manager.py
index fd2d65b6918b9fa316c28f06f0de353fea75db97..f5ada06fe447f3fec72da657484e934e3c8d879f 100644
--- a/djblets/extensions/manager.py
+++ b/djblets/extensions/manager.py
@@ -140,7 +140,11 @@ class SettingListWrapper(object):
     def remove_list(self, items):
         """Removes a list of items from the setting."""
         for item in items:
-            self.remove(item)
+            try:
+                self.remove(item)
+            except ValueError:
+                # This may have already been removed. Ignore the error.
+                pass
 
 
 class ExtensionManager(object):
@@ -616,7 +620,7 @@ class ExtensionManager(object):
             self.dynamic_urls.remove_patterns(
                 extension.admin_site_urlpatterns)
 
-        if extension.has_admin_site:
+        if hasattr(extension, 'admin_site'):
             del extension.admin_site
 
         self._context_processors_setting.remove_list(
@@ -851,7 +855,7 @@ class ExtensionManager(object):
                 self.dynamic_urls.add_patterns(
                     extension.admin_urlpatterns)
 
-        if extension.has_admin_site:
+        if getattr(extension, 'admin_site', None):
             extension.admin_site_urlpatterns = patterns('',
                 (r'^%s%s/db/' % (prefix, extension.id),
                 include(extension.admin_site.urls)))
diff --git a/djblets/urls/resolvers.py b/djblets/urls/resolvers.py
index 0d73f58dbf56a50adf852fabccb6afa18dd4437b..8e939070cd76701163ee6d8cd083bf639cc10412 100644
--- a/djblets/urls/resolvers.py
+++ b/djblets/urls/resolvers.py
@@ -68,7 +68,11 @@ class DynamicURLResolver(RegexURLResolver):
         These patterns will no longer be able to be looked up or reversed.
         """
         for pattern in patterns:
-            self.url_patterns.remove(pattern)
+            try:
+                self.url_patterns.remove(pattern)
+            except ValueError:
+                # This may have already been removed. Ignore the error.
+                pass
 
         self._clear_cache()