I just encountered one of those really strange bugs that comes along so rarely they're worth a blog post! One of my users in SharePoint 2010 complained that they could no longer reach their document library. They thought permissions might be at fault.
When I went to the site and clicked the navigation link for the library, instead of seeing the default view I got a download prompt to save a file called "AllDocuments.zip".
Initial assumption was that the navigation link was somehow wrong, so I deleted it and re-created it. Same issue.
Next I entered the link to the library view (there was only one view) manually, and appended '?contents=1' to the end of the url. This opened the page in web part maintenance mode. I used this screen to close the View Web Part on the page. Clicking to go back to the view page gave me the "AllDocuments.zip" prompt again.
Time to break out the big guns. Opening the site in SharePoint Designer 2010, I went to Lists and Libraries, and clicked on the offenbding library. I created a new View for the library and made it the default.
Finally then I could now go to the library without issue. I confirmed the old default view was still causing issues, and deleted it.
It seems to me that somehow the encoding for the page got corrupted, and convinced everything it was a zip file rather than a normal aspx page. Rare is a problem that you can't solve through the UI, cracking open SharePoint Designer is not something I have to do every day. Hope this helps someone!