diff --git a/src/werkzeug/datastructures/structures.py b/src/werkzeug/datastructures/structures.py
index 719ddbca76..69bfa2fa4e 100644
--- a/src/werkzeug/datastructures/structures.py
+++ b/src/werkzeug/datastructures/structures.py
@@ -129,6 +129,8 @@ def pop(self, key, default=_missing, type=None):
                     return None
                 return default
         try:
+            # This method is not meant to be thread-safe, but at least lets not
+            # fall over if the dict was mutated between the get and the delete.
             del self[key]
         except KeyError:
             pass