diff --git a/docs/source/contributing/conventions/test-coverage.rst b/docs/source/contributing/conventions/test-coverage.rst
index 038c0301cc..30fc39d19d 100644
--- a/docs/source/contributing/conventions/test-coverage.rst
+++ b/docs/source/contributing/conventions/test-coverage.rst
@@ -143,6 +143,14 @@ Running the Tests
    ``purge_modules``, which re-imported the package before every test and dominated
    the suite's time (:issue:`1065`). Both live in :file:`tests/_support.py`; pass
    ``restore=True`` when the class must leave :data:`sys.modules` as it found it.
+
+   A class whose tests each need their own import purges in ``setUp`` instead of
+   calling the helper, and says why in a comment there: ``ConstEnumRegisterFallbackTests``
+   in :file:`tests/const/test_const_enum_builtin_parity.py` does, because a lookup in
+   one test grows the registries' tables the next one reads. Calling the helper and
+   then purging the whole package, later in ``setUp`` or in ``tearDown``, throws the
+   class's import away while reading as if it kept it;
+   :file:`tests/project/test_per_class_import_guard.py` fails on that (:issue:`1538`).
 *  **Check ordering under plain unittest.** :file:`tests/conftest.py` heals cross-module
    import pollution after every test, so pytest alone cannot see it (:issue:`981`).
    :file:`util/run_unittest_leg.py` runs one :file:`tests/` subdirectory, then the
diff --git a/tests/protocols/transport/test_tcp_http_dispatch_unit.py b/tests/protocols/transport/test_tcp_http_dispatch_unit.py
index 46380c1eca..accffa766c 100644
--- a/tests/protocols/transport/test_tcp_http_dispatch_unit.py
+++ b/tests/protocols/transport/test_tcp_http_dispatch_unit.py
@@ -26,7 +26,7 @@
 import unittest
 import warnings
 
-from tests._support import purge_modules, time_limit
+from tests._support import reimport_once_per_class, time_limit
 
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
@@ -50,7 +50,7 @@ class TCPHTTPDispatchTests(unittest.TestCase):
     """TCP's port-80/8080 entries point at the proxy, and behave like it."""
 
     def setUp(self) -> None:
-        purge_modules(['pcapkit'])
+        reimport_once_per_class(self)
 
     def test_tcp_binds_the_http_proxy_on_both_http_ports(self) -> None:
         """Ports 80 and 8080 resolve to ``application.http``, not ``application.httpv1``.