Why Would default.aspx Be Missing from Default Documents in an IIS Site?

Is there a good reason (i.e., other than "somebody removed it") why default.aspx would be missing from the list of default documents in an IIS web site?

I just helped one of our support technicians fix a (brand-new) customer's installation of our web application. The application is based on ASP.NET MVC. This customer was running IIS 6 on Server 2003. The application was working fine if you typed in a full URI in the browser, but if you just attempted to navigate to the root, /, URI then you would see a 403 error.

I poked around long enough to discover that the default document property of the web site contained all of the usual suspects except for default.aspx. So I added that to the list, bounced IIS, and everything appears to work as usual.

But having this gone strikes me as unusual. I'm wondering what else might be "unusual" on this installation. Perhaps someone was just tinkering with IIS long ago, for reasons unknown. I'm wondering, though, if the customer has done something else that would cause this to go away, and if I should be aware of it. Is there a "common" reason why this file would be missing from the list?

From serverfault Craig Stuntz

• Some form of manual intervention is the culprit. Things don't just "go away". Either someone manually removed it (accidentally or not), a script was run against it to make it go away, or some kind of corruption (metabase funkiness or something) happened which I think is unlikely.

  Craig Stuntz : That's sort of what I'm asking: Is there a common script that people run to make this go away? Or a common reason for doing it? Or a tool which removes it? People sometimes randomly delete stuff, but I'm wondering if I can figure out why they did it, and what else they might have done.

  squillman : I don't know of any common script that does it. I have seen, though, times when it'll be removed from a site as a quick and dirty way to disable default.aspx as a default document during a maintenance window such that requests that trigger the default doc mechanism don't hit live code. Then default.aspx was not added back into the site. This is a pretty obvious fix, though.
  From squillman

• Default.aspx is not on the list on a fresh installation, you need to go add it first time.

  Craig Stuntz : I just looked on an untouched Vista box, and it's there. Don't have a virgin Server 2003 box at the moment to look at at the moment, though.
  From baldy