This is probably a mistake but it shows up in my cart. If you want it at that price you better hurry.

edit: It looks like people aren't able to checkout at that price. I was able to get to the checkout page but didn't go farther because I'm not interested int buying. Sorry.

edit 2: It appears lots of stuff (everything?) is showing 50% off so there's a problem with the store. The checkout system is offline for now. This applies to the web.

http://store.steampowered.com/app/115300/

Perhaps it was an error caused by the preparation for the Christmas sale? Not that MW3 should drop by that much, but they could have attempted a publisher bundle calculation and accidentally added it.