So, I just spent a good 6 hours trying to diagnose the strangest little quirk I was having. In one of our products, we use a Flash map selection product called TrueVector. This is implemented as a flash file (tvfm.swf) which itself grabs various configuration files (tv_cfg.xml) and xml data files from your web server in order to load up with a pretty little map you can select states and zip codes on. I have blogged about this troublesome control before (most of the trouble seems to be in getting all the files in the file structure and in the config file listings just right so that the flash successfully finds them).
The map solution was working fine, until I moved the page (and thus, out of necessity, some of the supporting files such as tv_cfg.xml) to a subdirectory in the web, specifically /Ads/campaign_edit.aspx. Now, the flash file would load up, but it would immediately display an error Failed to load main configuration data from: tv_cfg.xml. Now, the tv_cfg.xml file was in the same relative spot it was before in the root of the web page, and I actually had it in several other places just for testing. Firebug was not even showing a request for this file in the Net tab. To further confuse me, I tested this page in Internet Explorer, then in Firefox 2, then in Firefox 3 (where I was having the issue) on a virtual machine - they all loaded the map fine. I finally started getting close when I thought to try loading my development Firefox 3 in safe-mode, with all of the add-ons disabled. Now, it worked! Through the process of elimination, and to make a long story short, I finally figured out that AdBlock was causing the problem. I pulled up the Blockable Items (Ctrl-Shift-V) page and smacked myself on the forehead:
I could have just added an exception to AdBlock, but I decided instead to go through the work of renaming this directory in the website. That way, I can be more sure that if any of our customers use AdBlock or similar software, that they will not have issues with the site.