There are phrases that seem innocent until you examine who says them and when. If someone lies on some days and tells the truth on others, a single statement about yesterday can reveal—or rule out—almost every day of the week.
Monday's lie
Riddle statement
A person says:
—Yesterday I lied.
You know that this person always lies on Mondays, Tuesdays and Wednesdays, and always tells the truth on other days.
What day of the week could today be?