There are a couple ways to do this. The simplest would be to create a fill in the blank question and use the top (t) parameter to move the question off the screen (e.g., t2000). For question wording, instruct the subject to press "enter" to continue. This will bring up a message box that says something like "it would be ideal if you were to type something". You can edit the text of that message using a substitute.txt file (see user's guide for the section on using the messages.txt and substitute.txt files, or online here: http://www.empirisoft.com/medialab/help/preferences.htm).

You could change the text to "There has been a system error. Press enter to continue" or whatever you like. They can keep pressing enter and they will keep getting this message.

Slightly more complicated solutions might allow more flexibility (e.g., creating a custom html page) but this would be pretty quick to implement.