Note: Follow the madness here to it's conclusion, to discover yet another "Really? I didn't know that..." IE moment, that may save you heartache and pain.
Is the sarcasm evident in my title? It should be. While Internet Explorer may have been "groundbreaking" when it was released, it has ever been the bain of the web developer's existance. Why, you ask? Because it refuses to adhere to standards, and just operates differently than everyone else. To be fair, Internet Explorer 10 is in the works, and supposedly closes the gap a fair amount (and even wins out in some of the html 5/css 3 support), and 7, 8 and 9 did progressively improve (if slowly). Unfortunately the fact remains that some companies/organizations/governments are stuck on Internet Explorer 6, or maybe even 7. If you are a web developer, and write cross-browser web applications, chances are Internet Explorer has hit you more than once in your career.
It's the dumbest things that hit you too. The most obscure "WTF!?!" moments that drive you crazy. That is a daily experience for me now.