Multiple Class Selectors Don't Work in IE6

This is well-known knowledge, but I since it was not to me, I'll still catalogue it here. When you use a CSS selector with multiple class names on the same element, i.e.:

.class1.class2{
   ...
}

IE 6 will interpret this as:

.class2{
  ...
}

That's right, the first class will get ignored.