When I mentioned this on #clojure, @bitemyapp suggested I file an issue, although I don't see this as a real bug.
When doing silly things with includes and extends, I get a 500 server error after a long loading period (and a huge stacktrace.)
Easiest steps to reproduce:
$ lein new luminus foo cd foo && vi src/foo/views/templates/about.html
insert into ~line 3 (after extends)
{% include "foo/views/templates/about.html" %}
then
lein ring server-headless
and surf to localhost:3000/about
This also works the other way round iirc, if you extend something that's including the file you work on - maybe it would make sense to add an arbitrary recursion limit of ~10 levels deep.