I have experienced this bug two time in two different times. I am sorry I can't provide any screenshots or anything, these two times were circumstantial and not expected at all.
This bug consist when you merge two threads, and try to delete one of the posts (the first one and the second one were at least selected to be deleted), , and it deleted the whole thread!!!!!!!!!!!!!! ( it had like 10+ more posts on it )
I had to apologize to the members who had started these threads and it was very embarrassing to have to tell them their thread got deleted.