When checking in a file in VS against TFS, if you click on the Work Items section, select a work item, and change the Check-in Action dropdown by selecting it and pressing “a”, which signifies “Associate,” when you click off of the dropdown, the value is reverted back to the default, “Resolve.”
Reported @ https://connect.microsoft.com/VisualStudio/feedback/ViewFeedback.aspx?FeedbackID=260453
Edit: I'm not sure who responded to this, but they obviously have a problem with English -- the first sentence didn't even make sense. They seemed to think the problem only happens on one work item as an odd occurrence. They suggested I discuss this in the forums. The problem with that is that I've been able to reproduce this on all work items using multiple TFS instances.