To preface: this has got to be one of the strangest bugs I have ever seen, especially since it comes and goes.
There exists a page called view.php and another page called save.php. The bug manifests when I request save.php - I instead get view.php. Request headers say save.php, and it happens in Firefox, IE, Chrome, Opera, Safari. It happens consistently unless - and here's the strange part - I open up the file and save it. I don't make any changes to it, just save. After saving, I can make the same exact request and it serves up save.php like nothing was ever wrong.
I'm currently exporting from an svn repository (just a simple svn export http://server/repository target command). If I export after not making any changes, the bug re-manifests. If a make a change (to an entirely unrelated page), and commit it to the repository, then export, the bug usually goes away. However, the same thing might happen with two different pages (also unrelated to the changed page), or it may not.
I'm not using any kind of caching (no php caching, browser caching, or apache caching).
SVN versions: 1.6.9 w/ AnkhSVN on a Windows machine (the development machine), 1.4.2 on both the repository machine and the test machine (where I run the export command).
Aside from suspecting svn is where something is going wrong, I have no idea.