Mary Wakefield says that she doesn't lie, but says that makes her a liar.

There are a few gags involving lying on The Simpsons, and here's one. This is when the kids were going to the dentist to have their teeth checked. Lisa needed braces, but since Homer's union gave away the dental plan, she couldn't get them. Some throwaway bits from that episode. Dr. Wolfe is the dentist.

Dr. Wolfe: How often to you brush Ralph?
Ralph Wiggum: Three times a day sir.
Dr. Wolfe: Why must you turn my office into a house of lies?

And later, Wolfe inspects Maggie's baby teeth.

Dr. Wolfe: Maggie's teeth are coming in rather crooked. Has she been sucking on a pacifier?
Marge: Errrrrmmmmm... Not to my knowledge.
Dr. Wolfe: Liar!