I've got this in /etc/pihole/adlist.list (copy of /etc/pihole/adlist.default):
# My local list
http://localhost/mylist.txt
I ran pihole -g, but (stupid me) the file /var/www/html/mylist.txt wasn't there yet, so the result list (/etc/pihole/list.13.localhost.domains):
<html>
<head>
<script>window.close();</script></head>
<body>
<img src="data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7">
</body>
</html>
I noticed the error, so I copied the file (/var/www/html/mylist.txt) from my backup
However pihole -g failed to update the result (unchanged result file)
I could resolve the issue by deleting /etc/pihole/list.13.localhost.domains and running pihole -g again
The lighttpd access log showed the following:
1489583802|localhost|GET /mylist.txt HTTP/1.1|200|179
1489583987|localhost|GET /mylist.txt HTTP/1.1|304|0
This problem can be duplicated by deleting the result file (/etc/pihole/list.13.localhost.domains) and removing the source (/var/www/html/mylist.txt)
Looks like error 304 requires some special error handling.