Note the ImportRange can cause a security violation. You can use the function to draw data from one of your spreadsheet not shared with others. However when you share the spreadsheet with the ImportRange imported data, the collaborators/viewers of that sheet get access to data they are not entitled to.

The Google Docs team should enforce a re-evaluation of the ImportRange function whenever someone opens the spreadsheet. Re-evaluation shows the ImportRange function to conform to the sharing rules; when the user is not entitled accessing the imported data it won't show.

Until this has been repaired this the ImportRange data is to be considered unsafe when working with sensitive data