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

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…)