Django URL resolving infrastructure stops working
This has happened to me before. Normally it's due to a 'broken' urls.py file. There are two things that make this kind of bug really hard to fix:

It could be the urls.py file in any of the apps that breaks the reverse() function, so knowing that reverse() breaks for app X doesn't mean the error is in that particular application's urls.py.
Django won't even notify you of errors in the urls.py file, unless you somehow manage to crash the site by doing something really, really nasty in the urls.py file.

Debugging: The way I go around fixing this is by manually disabling all applications (just comment out their line in INSTALLED_APPS) and checking reverse() works. If it still works, then I enable the next app, until it breaks. I know, very rudimentary stuff, but it works :)
We recently launched a new Django-powered website, and we are experiencing the oddest bug:
The site is running under Apache with mod_fastcgi. Everything works fine for a while, and then the URL tag and reverse() functionality stops working. Instead of returning the expected URL, they return "". 
We haven't noticed anything in Apache's log file; there are no errors being generated by Django. And (the kicker) the problem only occurs in production mode; we can't reproduce it when DEBUG=True. 
Any thoughts on where we should be looking for the problem?
Update: It turned out to be a problem with settings.LANGUAGES, although we haven't determined exactly why that broke things.