I was sent a word document with "hidden data" in it.
"Hidden data" in Microsoft Word documents can be one of several bits of information, such as hidden data, change tracking, or even comments on the document.
Comments.
Such as this example: 
This problem isn’t just limited to Microsoft Word, oh no. It shows up in Microsoft Excel and PowerPoint too.
The solution for:
Microsoft Office XP/2003? Download the Microsoft Remove Hidden Data add-in, and use it.
Microsoft Office 2007? You already have it. The option can be found under the Office Pearl –> Prepare –> Inspect Document menu item. 
This post brought to you by the outside company who shared a comment-enabled document with me.
On this day..
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There are tools that remove hidden data straight from the outbox, like Workshare Protect and ConfidentSend.
That would not be a bad idea. I mean, if the user forgets to remove the “hidden data”, at least the outbox scanner would trap it.