We've been using Ruby on Rails for our development efforts for a while now, including for our website. We recently added some PNG graphics, utilizing semi-transparent shadows, which looked great on Firefox/Safari. On IE, however, we know they would look terrible. So we added one of the publicly available scripts that fix the problem. There was only one issue: it didn't work. At all.
After putzing around with different versions of the PNG fix script, and having none of them work, I finally started debugging. The scripts were being called, but never finding any PNG images to modify. So, it finally became apparent: the script I was using was looking for any IMG tag whose "src" property ended in ".PNG". Well, we use Rails. And Rails, by default, appends a randomized number to the end of URLs to defeat IE's URL-based caching mechanism. This means that none of our images ever matched the PNG script's test.
The solution was simple. Change:
if (imgName.substring(imgName.length-3, imgName.length) == "PNG")
and suddenly everything works.
An example of the law of unintended consequences (appending randomized numbers to every URL breaking a fix for non-transparent images) crashing into the law of unanticipated difference (don't ALL image "src" attributes end in .png, .gif or .jpg?) Like a Reese's Peanut Butter Cup of Doom.