WordPress has had a long standing issue with efficiency in some of its handling of shortcodes. Depending on how you look at it, this could be considered more of an issue with php.
This manifests itself in Simple:Press that on occasions when you make a new post in a topic, the topic essentially disappears when you load it. Removing the last post will cause the topic to reappear. The problem is that the PHP Backtrack Limit has been exceeded. Luckily, there is a simple fix you can make in your wp-config.php file to increase this limit. Just add the following line to your wp-config.php file:
Some users have had to add a couple of extra zeros to that number, so don’t be afraid to try that.
For reference, you can review the WP Trac Ticket on this issue which has been open for quite some time.