Examples of overstatement?
"Overstatement" basically means to exaggerate something, or say that something is greater than it actually is; give something too much credit. Understatement is the opposite; not giving enough credit.
Example of understatement: "He performed okay." when talking about someone who performed excellently, hence the usual response "That's an understatement!"
Example of overstatement: "He is an incredible artist." when talking about someone who is an average artist at best, hence the usual, yet rude response "That's an overstatement!"