Came across a nasty little bug today1 whereby IE6 incorrectly displays a nested, z-index’d div below the content it is supposed to be above.
For the uninitiated, z-index basically makes your two dimensional screen three dimensional by adding a Z-axis. If you’re not sure what a z-axis is then pay more attention in geometry class.
An article entitled CSS: z-index – so what appears on top?.
1 This was actually quite some time ago now but I just found the link carelessly lying around in my draft box.