I just learned the hard way that IE8 in IE7 compatibility mode is not the same as Internet Explorer 7. Apparently IE7 compatibility mode uses the IE8 JavaScript engine and you won’t be able to detect IE7-only JavaScript bugs when testing and developing in compatibility mode.

I recommend using IETester, it will throw all the errors you ever wanted (NOT!) at you. Combine it with Firebug Lite and sleep a little better.