One of our development sites was having trouble rendering in IE 6. It was previously browser tested-- everything checked out! Suddenly a tester comes back with the infamous comment (you might know it well): "the site doesn't render properly in IE6".
For most of us, this means several hours pouring over code trying to root out the problem CSS. If you are on a Mac, or a PC which doesn't allow IE6, this is especially painful.
I realized that IE6 wasn't loading the CSS file when I was able to copy and paste all of Zen's built-in (and rather impressive) IE specific CSS fixes and load the site properly. After about 2.5 hours of painstaking code glaring, I spotted this helpful hint.
Basically, stupid evil IE6 can't load more than 31 css files, which is basically the minimum number for Zen and most modern Drupal themes. The solution takes under 1 minute to fix: turn on CSS compression in the Performance section.
If you're reading this, you may have already spent over a couple hours tracking it down-- I feel your pain, take a break, enjoy your day, smell some flowers and remember that soon IE6 will be a distant memory.
Thanks dude