Well, I did this in class and it seemed like a good idea to put on a forum. It's a short mystery, it has to be confusing, and the person who submitted it needs to ask one of these questions; "What is his/her occupation?" "What was he/she getting/doing?" "Where was he/she going?" "Why did he/she do that?" and so on. If people get stuck, don't be afraid to give out small hints like this:
"She was using only 2 quarters..."
And so on. So come and send in your 30 second mysteries and LET THE GAMES BEGIN!