If you insert a bookmark in a rich text content control, the bookmark may disappear after saving and reopening the document.
This issue appears to be a problem with Word 2007 and Word 2010. To work around the issue, insert the content control before typing the text. See the Resolution section for an example of how to retain the bookmark in a Content control.
Insert a 2x2 table
In any cell, insert a rich text content control (Developer Tab)
Place the IP in the content control and type "Line one"
At the end of the content control and type Shift+Enter to insert a new line
Type "Line two"
Select the text from step 6 and insert a bookmark around it (Insert Tab)
Save, close and reopen the document
Go back to the Insert Tab and click on the Bookmark dialog
The Bookmark is retained.