diff --git a/tests/test-run-tests.t b/tests/test-run-tests.t
--- a/tests/test-run-tests.t
+++ b/tests/test-run-tests.t
@@ -1,5 +1,12 @@
 This file tests the behavior of run-tests.py itself.
 
+Avoid interference from actual test env:
+
+  $ unset HGTEST_JOBS
+  $ unset HGTEST_TIMEOUT
+  $ unset HGTEST_PORT
+  $ unset HGTEST_SHELL
+
 Smoke test
 ============