My presentation was well received by the nearly 800 attendees today so I am in a terrific mood! (I've nearly forgotten the United Airlines abuse)...so here is your hip tip as promised...
AutoCAD Hip Tip to keep others from editing your Xref'd drawing:
Since AutoCAD 2004 you have the added ability to double click on an Xref and edit it in place. This literally opens up the drawing file and allows you to make changes. I recently received an email from a reader who was wondering if there is some way to turn this feature off - he was tired of people making changes to his xrefed drawings without realizing the impact.
So here is your answer - in the drawing you wish to protect - set the system variable XEDIT to "0".
So there's your Hip Tip from London - I'm off to catch a plane to Ireland!