I regularly encounter this error as well. I am using the latest version - 1.0.9
I wonder if the issue is that between the time that a guest submits their post and when they actually login (and the post actually gets submitted) someone else has posted in the thread. If so, a potential hack that would fix it is to not calculate the post timestamp and post_id until the actual insertion point, after the user has successfully logged in. I haven't looked through the code, so it may already do this, I don't know.