diff --git a/_pytest/hookspec.py b/_pytest/hookspec.py
index 3f1e7d022..d0c3e4aad 100644
--- a/_pytest/hookspec.py
+++ b/_pytest/hookspec.py
@@ -30,8 +30,15 @@ def pytest_plugin_registered(plugin, manager):
 def pytest_addoption(parser):
     """register argparse-style options and ini-style config values.
 
-    This function must be implemented in a :ref:`plugin <pluginorder>` and is
-    called once at the beginning of a test run.
+    .. warning::
+
+        This function must be implemented in a :ref:`plugin <pluginorder>`
+        and is called once at the beginning of a test run.
+
+        Implementing this hook from ``conftest.py`` files is **strongly**
+        discouraged because ``conftest.py`` files are lazily loaded and
+        may give strange *unknown option* errors depending on the directory
+        ``py.test`` is invoked from.
 
     :arg parser: To add command line options, call
         :py:func:`parser.addoption(...) <_pytest.config.Parser.addoption>`.