From 454134e21295fb9da4e74980360d89c287ef02dc Mon Sep 17 00:00:00 2001
From: Matthew Silverman <matthews@xanadu.ai>
Date: Tue, 4 Jul 2023 14:49:23 -0400
Subject: [PATCH] add warning about applying a queued operator (#4311)

* add warning about applying a queued operator

* move warning into warning block
---
 pennylane/queuing.py | 16 ++++++++++++++++
 1 file changed, 16 insertions(+)

diff --git a/pennylane/queuing.py b/pennylane/queuing.py
index 1b99b5f8328..e2c84602e0e 100644
--- a/pennylane/queuing.py
+++ b/pennylane/queuing.py
@@ -459,6 +459,22 @@ def circuit(x):
     >>> print(qml.draw(circuit)(0.6))
     0: ──RX(0.4)──RY(0.6)──RX(0.4)──┤ ⟨Z⟩
 
+    .. warning::
+
+        If you use ``apply`` on an operator that has already been queued, it will
+        be queued for a second time. For example:
+
+        .. code-block:: python
+
+            @qml.qnode(dev)
+            def circuit():
+                op = qml.Hadamard(0)
+                qml.apply(op)
+                return qml.expval(qml.PauliZ(0))
+
+        >>> print(qml.draw(circuit)())
+        0: ──H──H─┤  <Z>
+
     .. details::
         :title: Usage Details