Add docs for useRef no no to fix double effects

src/content/learn/synchronizing-with-effects.md

@@ -598,6 +598,33 @@ Usually, the answer is to implement the cleanup function.  The cleanup function
 
 Most of the Effects you'll write will fit into one of the common patterns below.
 
+<Pitfall>
+
+#### Don't use refs to prevent Effects from firing {/*dont-use-refs-to-prevent-effects-from-firing*/}
+
+A common pitfall for preventing Effects firing twice in development is to use a `ref` to prevent the Effect from running more than once. For example, you could "fix" the above bug with a `useRef`:
+
+```js {1,3-4}
+  const connectionRef = useRef(null);
+  useEffect(() => {
+    // 🚩 This wont fix the bug!!!
+    if (!connectionRef.current) {
+      connectionRef.current = createConnection();
+      connectionRef.current.connect();
+    }
+  }, []);
+```
+
+This makes it so you only see `"✅ Connecting..."` once in development, but it doesn't fix the bug.
+
+When the user navigates away, the connection still isn't closed and when they navigate back, a new connection is created. As the user navigates across the app, the connections would keep piling up, the same as it would before the "fix". 
+
+To fix the bug, it is not enough to just make the Effect run once. The effect needs to work after re-mounting, which means the connection needs to be cleaned up like in the solution above.
+
+See the examples below for how to handle common patterns.
+
+</Pitfall>
+
 ### Controlling non-React widgets {/*controlling-non-react-widgets*/}
 
 Sometimes you need to add UI widgets that aren't written to React. For example, let's say you're adding a map component to your page. It has a `setZoomLevel()` method, and you'd like to keep the zoom level in sync with a `zoomLevel` state variable in your React code. Your Effect would look similar to this: