Those that pre-ordered Left 4 Dead 2 will have access to an exclusive crossover item for Team Fortress 2: Bill’s Hat will be available today with the Team Fortress 2 update, which also fixed client-side perf problem after changing levels on Koth_Harvest_Event.

But why? TF2 Blog lends us a little history: “And lest you think this is just some marketing gimmick, here’s the intricate back-story justifying it: while conducting a science experiment on parallel dimensions, Bill’s hat fell off a table into the Team Fortress universe.”

If you haven’t preordered Left 4 Dead 2, you can still do so and get Bill’s hat for TF2, and other pre-order goodies.