I have created a simple block as follows:
<div style="float: right;">%%div_contents%%</div>
I had to use the editor in HTML mode else it'd "interpret" the div and would also add extraneous nbs and <p>.
Later on, I type this in the editor:
[contentblock id=1 div_contents="[slideshow]"]
Upon saving the post or switching between HTML and Visual, I get a broken content block image that is missing the last two characters: "] and the same two characters appear next to the right of the content block image.
Saving and viewing the page shows the whole content block doing nothing and all you see are those "] character.
I don't know if this has anything to do with this WP shortcode API notice:
The shortcode parser does not accept square brackets within attributes. Thus the following will fail:
[tag attribute="[Some value]"]