[Fixed]-What could cause a Django error when debug=False that isn't there when debug=True

Just to say, I ran into a similar error today and it’s because Django 1.5 requires the ALLOWED_HOSTS parameter in the settings.
You simply need to place this row to make it work 😉

...
ALLOWED_HOSTS = '*'
...

However, be aware that you need to set this parameter properly according to your actual host(s) (https://docs.djangoproject.com/en/dev/ref/settings/#allowed-hosts)!

Values in this list can be fully qualified names (e.g. ‘www.example.com’), in which case they will be matched against the request’s Host header exactly (case-insensitive, not including port). A value beginning with a period can be used as a subdomain wildcard: ‘.example.com’ will match example.com, www.example.com, and any other subdomain of example.com. A value of ‘*’ will match anything; in this case you are responsible to provide your own validation of the Host header (perhaps in a middleware; if so this middleware must be listed first in MIDDLEWARE_CLASSES).

So basically it’s better for you to use this type of configuration once you’re in production:

...
ALLOWED_HOSTS = [
    '.yourdomain.com',
]
...

thanks to gertvdijk for pointing this out

This can also happen if you do not have both a 500.html and 404.html template present. Just the 500 isn’t good enough, even for URIs that won’t produce a 404!

I had this problem as well. Although it persisted even when setting Allowed_hosts and already having 404 and 500 templates.

I also checked for circular imports, but that was not it.

I finally had django produce a log file, https://stackoverflow.com/a/15100463/1577916

I accidentally left in a “get_host” function which now exists under
HttpRequest (changed to HttpRequest.get_host())with Django 1.5.

for some reason that was not raising an error with Debug True OR False.