Text input ghost image

I'm just taking a shot at this because there's not much to go on.

I am aware of a similar complaint regarding invisible buttons with a shortcut key assigned to them. When an invisible button is triggered via a keyboard shortcut, that invisible button ends up getting a yellow rectangle on it -- something to do with Captivate's "focus manager," or some such.

Could it be that you have a single slide with multiple text input fields on that slide?

If so, I can imagine a variety of problems (including the yellow box or "ghost characters") being exhibited.

The most effective way to build interactions in Captivate is to use only one interaction per slide. If you have one screenshot on which you need to simulate the entry of text in multiple fields, you'll be better off using a separate slide for each field:

1) On slide "a", have the student enter the text in field 1.
2) On slide "b", display the correct answer in field 1, and have them enter text in field 2.
3) On slide "c", display the correct answers in fields 1 and 2, and have them enter text in field 3.

Repeat this "build up" of entries until they've made all the necessary entries.

I suspect if you build your interaction in this manner, the "ghost" issue you're seeing won't show up at all. If this doesn't work for you, maybe you can describe your situation better or provide an online sample we can look at.