A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
Bingo. That's why I couldn't repro it at first. I wasn't clicking directly into the "target" text box right away.
I was hoping that saving/reopening the presentation would toss out these blank text boxes but no such luck. If you have a lot of them and they're causing trouble, here's some VBA you can beat it with. This'll look at each shape on each slide and if it's one of these duds, should delete it.
Sub TakeOutTheTrash()
Dim oSl As Slide
Dim oSh As Shape
Dim x As Long
For Each oSl In ActivePresentation.Slides
For x = oSl.Shapes.Count To 1 Step -1
Set oSh = oSl.Shapes(x)
If oSh.HasTextFrame Then
If oSh.TextFrame.HasText Then
Debug.Print oSh.TextFrame.TextRange.Text
If Len(oSh.TextFrame.TextRange.Text) = 0 Then
oSh.Delete
End If
Else ' has text frame but no text
oSh.Delete
End If
End If
Next ' Shape
Next ' Slide
End Sub