Be made a sitter for someone
Sit for the maximum amount of time
At the end of the period only the the person that you are sitting for can dismiss you, you can't dismiss yourself as options can't be accessed.
I have just come across this while sitting for someone, as the period expired I wasn't able to dismiss myself, solved the problem by contacting the person I was sitting for, however, it would become a problem if the user becomes inactive and I find myself with a slot that I can't free.
See the screenshot below, options are greyed out.