diff --git a/oonipipeline/src/oonipipeline/transforms/measurement_transformer.py b/oonipipeline/src/oonipipeline/transforms/measurement_transformer.py
index 11243f85..ec1b013a 100644
--- a/oonipipeline/src/oonipipeline/transforms/measurement_transformer.py
+++ b/oonipipeline/src/oonipipeline/transforms/measurement_transformer.py
@@ -48,6 +48,7 @@
 
 log = logging.getLogger("oonidata.transforms")
 
+# see also: https://github.com/ooni/probe/issues/2420
 unknown_failure_map = (
     (
         "A socket operation was attempted to an unreachable network",