The really funky part of the IE6 PNG fix was that the URL of the image in the expression was always relative to the current page URL, not the CSS file that the declaration was in.

I remember having to use Javascript to generate the image path (I cannot quite remember why absolute or root-relative URLs were not an option for us though...)

I built a mildly responsive site in 2007 using transparent PNGs overlaying a photo in the header, only for it to turn out that the client’s IT policy blocked the PNG loader. In the end I had to explain to them that everyone else would see the site normally, and they’d have to put up with it until IE7 launched.