diff --git a/newsfragments/1599.deprecated.rst b/newsfragments/1599.deprecated.rst
new file mode 100644
index 0000000000..2d0ecc6bf6
--- /dev/null
+++ b/newsfragments/1599.deprecated.rst
@@ -0,0 +1,19 @@
+In the current implementation of `nursery.start()
+<trio.Nursery.start>`, the new task starts running in a private
+nursery created inside :meth:`trio.Nursery.start`, and only moves to
+its "real" nursery once it calls ``task_status.started()``. Since Trio
+doesn't hand out any references to that private temporary nursery, we
+would like to be able to assume it only contains the single task being
+started. However, the nursery object is still accessible using
+debugging APIs such as
+``trio.lowlevel.current_task().parent_nursery``, and it's possible
+that some users have been using those APIs to start tasks in it. (If
+it wasn't clear, we strongly recommend against this sort of thing,
+because it makes private implementation details affect the behavior of
+your code.)
+
+Until now, these sorts of shenanigans unintentionally *worked*: the
+new tasks would move alongside the task being ``start()``\ed, and one
+might not even realize they weren't getting the ``parent_nursery``
+they were expecting. Supporting this unintentional feature is becoming
+burdensome, though, so we're deprecating it with no replacement.
diff --git a/trio/_core/_run.py b/trio/_core/_run.py
index 8b112051e5..3db4be8989 100644
--- a/trio/_core/_run.py
+++ b/trio/_core/_run.py
@@ -41,7 +41,7 @@
 )
 from ._thread_cache import start_thread_soon
 from .. import _core
-from .._deprecate import deprecated
+from .._deprecate import deprecated, warn_deprecated
 from .._util import Final, NoPublicConstructor, coroutine_or_error
 
 _NO_SEND = object()
@@ -656,6 +656,20 @@ def __repr__(self):
     def started(self, value=None):
         if self._called_started:
             raise RuntimeError("called 'started' twice on the same task status")
+        if len(self._old_nursery._children) > 1:
+            # This can only happen if someone finds the old_nursery through
+            # debugging APIs and spawns more tasks into it. We don't think
+            # that's a reasonable thing to do, and supporting it makes other
+            # extensions to the nursery semantics much harder to implement.
+            # But it unintentionally worked in the past, so it gets a
+            # deprecation period.
+            warn_deprecated(
+                "starting additional tasks in the private old_nursery created "
+                "by Nursery.start()",
+                version="0.16.0",
+                issue=1599,
+                instead="a nursery that you created",
+            )
         self._called_started = True
         self._value = value
 
diff --git a/trio/_core/tests/test_run.py b/trio/_core/tests/test_run.py
index 78b46b7adc..b818174d81 100644
--- a/trio/_core/tests/test_run.py
+++ b/trio/_core/tests/test_run.py
@@ -23,6 +23,7 @@
 )
 
 from ... import _core
+from ..._deprecate import TrioDeprecationWarning
 from ..._threads import to_thread_run_sync
 from ..._timeouts import sleep, fail_after
 from ...testing import (
@@ -1883,6 +1884,16 @@ async def raise_keyerror_after_started(task_status=_core.TASK_STATUS_IGNORED,):
         await closed_nursery.start(sleep_then_start, 7)
     assert _core.current_time() == t0
 
+    # started() raises a deprecation warning if there's more than one
+    # task in the old_nursery
+    async def starts_another_task_in_old_nursery(task_status):
+        _core.current_task().parent_nursery.start_soon(_core.checkpoint)
+        task_status.started()
+
+    async with _core.open_nursery() as nursery:
+        with pytest.warns(TrioDeprecationWarning, match="private old_nursery"):
+            await nursery.start(starts_another_task_in_old_nursery)
+
 
 async def test_task_nursery_stack():
     task = _core.current_task()