In some HTML text input fields a horizontal blip appears on top of the cursor when you clickon the field. See attached screen shot from the search box in the upper right corner of http://www.adobe.com/. Build 2006070201 on Mac OS X 10.4.7 PPC (8J135).
That's the bidirectional input cursor, and is the expected appearance (though admittedly it can be startling).
Not invalid (but a dupe)
*** This bug has been marked as a duplicate of 288839 ***