A frequent pattern in TFS/Team Build is to merge from one branch to another using a label as the basis for the merge.  (That is, you select a label in the source branch that designates the point you want to merge "from".)  Often, this label was applied by Team Build automatically.

This might play out like: "I know this build of this feature branch is good; I'll use the corresponding label as the basis for a merge back to the trunk."  Etc.

If this sounds like you and your shop, be sure to enable the feature that Buck Hodges discusses here to make sure that your build label sticks around even when your retention policy indicates the corresponding build should be deleted.  Otherwise, if the merge process takes awhile (due to conflict resolution, or lunch) you might find that upon completion of your work, you get an error indicating the label you were using cannot be found.

If this scenario does play out poorly for you...you could attempt to deduce the time at which your build label was applied and then apply your own label (with the same name) to that point in time on the source branch.  The merge process will then complete...

(No, really, I didn't get burned by this...)