Every so often I'll reopen a .graffle to find that one or more of the canvases are "missing". They still show up in the canvases list on the left, but the thumbnail is sort of shifted left and clicking on it won't update the view on the right. From what I can tell, its always a canvas that has an image pasted into it. Once opened, it also causes OmniGraffle to act strange, for example resizing the app window redraws rulers all over itself. I thought it might be a Dropbox issue but I am now seeing the bug in files on my Desktop. I can usually restore via Time Machine but its strange and frustrating.