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:
first gap;second gap;number three;another
(You'll see in the picture above that 5 red herrings have been added).
This adds some extra formatted text (###redherrings:first gap;second gap;number three;another###) to the end of your text in the "Text" and "Show all" boxes.
Now, when you view the user-defined gap-fill - with the word list showing - these extra "red herring gaps" will appear as options for the gaps in your exercise.
N.B.: These red herrings only appear if you choose to view the word list.
You can choose to view the word list on the activity screen itself, by clicking on the "Show words" button.
Or if you want to link to an activity or embed it somewhere else, you can tweak the activity url by adding e11 or e10 to the activity url (e11 = show word list, autocorrect on; e10 = show word list, autocorrect off). These url tweaks are explained on this really useful blog post.
(You can also select to show the word list if using the gap-fill as part of a sequence.)
Here is a link to the gap-fill from the image above, with the word list showing (and autocorrect off): http://www.textivate.com/udgapfill-o0ljn1e10
And here is the same activity embedded, this time with autocorrect on:
Adding red herrings to your gap-fills makes them more challenging, as students know that there are words that are not used in any of the gaps, so they can't simply fill in unknown gaps by a process of elimination.
Of course, you can print worksheets with this new feature too. Here's a gap-fill worksheet based on the above text: