It sometimes appears in Illustrator CS6, only with some text:
It's annoying me. How can I remove it?
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Sign up to join this communityShows the text is longer than the text frame area. You have two possibilities:
This means that the text exceeded the size of the container. You need to make the container larger so that all the text fit in it, or link it to another text container (click on the red square and then on another text frame).