When designing a page the page will look allright in firefox however it wont in IE. Its always IE that goes on a mad one first, then when you try fixing it for IE then check it in firefox it would look a mess, so you would attempt to fix it for firefox then IE fucks up again.
Not sure about IE7 but in IE6 this is a nightmare in a half, if you miss one bit of code in the layout side of things your banging your hed off a brick wall trying to figure out why it wont align properly etc