These are sometimes extremely difficult to call.
@MPD, you ask for examples.
I think the one you probably refer to (the one closed as off-topic, and then reopened again) is an excellent example: Adding .htaccess within httpd.conf correctly.
The question looked to most of (including me) as something that only involved Apache (i.e. off-topic). However, when it was re-opened, the first (and so far only) answer was Drupal-centric (i.e. the criterion: "Does the answer change if you use WordPress instead of Drupal" can be answered in the affirmative - and that answer also could only be given by someone familiar with both Drupal's
.htaccess files [plural!] and Apache). You probably would not get such a Drupal-centric answer if you asked this on SO, Server Fault, or Pro Webmasters.
If the quoted criterion is going to be set as an official criterion for closing or not (as is implicit in the suggestion by kiamlaluno), I think we need to be wary about closing stuff as off-topic too quickly. Give them at least 24 hours. You cannot really judge if the answer change if you use WordPress instead of Drupal until you've actually seen the answer. Closing them prevent answers from being posted. Sometimes somebody comes up with a really good Drupal-centric answer to something that on first glance seems unrelated to Drupal.
If we take "Server administration/deployment" out of the list of allowed topics, it will become far easier for the topic-vigilantes among us to get these closed (too) fast.
I am in favour of keeping it in the list, but close as off-topic those that after the community has had a reasonable time to respond (I suggest 24 hours) appears to not have anything to do with Drupal.