The rules is similar to finish the sentence but theres a difference...
Basically you have to take sentence started by someone else and turn it into bad news.
Please don't just repeat same bad news, like always finishing a sentence with "... but a bomb went off!".
Example:
Previous post sentence start:
At school library, I was browsing a...
Post completing (and turning the sentence to a bad news):
website, but amazing number of porno popups appeared!
This should be straightforward enough. 
Just try to be reasonable setting a new sentence so other person can finish it.
First sentence to be finished:
When I was hiking, I saw such a...
