Using the plugin post preview, I came across a nasty little thing several times now:

• if you embed an image in your post being edited and you preview the post and – by mistake – click on the image in the preview area, you get it zoomed up in a thickbox – and no way back, because the close button of the thickbox is missing. I’ve tried several ‘hot keys’ , like ‘Esc’, ‘ctrl c’ , aso  – but no success. You can’t get rid of the image.

Going back by reloading the page destroys the complete content of your editor and you start from scratch.

I would suggest to either do not accept clicks on images in the preview area or add a close button to the thickbox if popping up from preview area.