IMPORTANT: This is actually NOT a 403 error, it's a 500 error but the custom html error page is not displaying the right message [see T229402]
Steps to re-create the 403 error on live:
- Make sure you are logged in to OAuth
- Navigate to any page
- Wait for around 15 mins (still testing to get exact time)
- Try to navigate to any page of the app (even the page you were already on)
At this point the error shows and can persist for another few page navigations/refreshes until it goes back to normal.