Leave Debug on in Django for internal application
Debug mode might leak a bit of memory. Additionally, it is much better for production systems, however small, to email their administrator with the full error message and stack trace (which Django does by default when DEBUG=False) than to show it on the browser. This way the administrator knows exactly what happened instead of trying to reproduce it with vague information from the users ("I clicked here and then I think I clicked there and then there was this message"). You need to set the ADMINS and EMAIL_* settings correctly though.
I'm working on a Django project that I took over for someone else that is only used internally.  It's not deployed to a website and can only be accessed on a local network.  The previous developer had left DEBUG = True in settings.py.  Django docs really emphasize that leaving DEBUG=True when the site is in production is bad.  The site is inaccessible by anyone not on the local network, and is only even looked at by ~5 people regularly.  Aside from security reasons, is there any other downside to operating permanently in DEBUG mode?