So what gives? Well, the staging environment was serving SWFs with gzip compression and this was messing up the bytesLoaded and bytesTotal properties that my preloader SWF was trying to read from the main SWF. Different browsers reacted differently too. Some saw bytesTotal as 0 (IE), which is what gave me the infinity percentage (dividing bytesLoaded / 0 will give you Infinity), while others reported bytesTotal as the same as bytesLoaded (Firefox). Hard coding a fake bytesTotal was my temporary fix.
To check to see how your server is sending your files, you can use Charles to inspect your web traffic. Here’s the one that was breaking because of gzip compression:
And here’s what it should look like:
I originally thought the issue was with the SWF being served with the wrong MIME type of “text/plain” but changing that didn’t fix my problems. The moral of the story is to check these server settings before wasting a Saturday (Gasp? Working on a weekend?) trying different versions of a Flash preloader and waiting for staging to build the site after each try.