If you make a user-defined gap-fill on textivate (as explained here and here) you can now add "RED HERRING" words to the word list.
These are words that cannot fit into any of the gaps that you have selected and they make the exercise more challenging for the student.
To add red herrings, first create your user-defined gap-fill by clicking on the "User-defined gapfill" button on the textivate home page:
With the user-defined gap-fill edit screen open, scroll down to beneath the box containing your text and you should see a section like this:
You can add extra words (or groups of words) into this box, separating your red herrings with semi-colons, like this: