A rip is when someone says or does something that turns out to be wrong. When they do, you can say "That's a rip."
Depending on how it works in your area, you can also inflict pain to the people who got ripped.
Joe: John's in the steakhouse, I swear!
*John walks out of a supermarket*
David: ...That's a rip.
A rip (not to be confused with rip-off) is a slang term which can refer to anything comical, hypocritical, or embarrassing in nature. The term grew popular in Northern California in the early 2000s and eventually became common Norcal schoolyard slang along with hella, hecka, and the term Norcal itself. Its usage is commonly accompanied with a rough "stripping" motion on the back of one's neck, as if removing the tag on his/her shirt. Its usage continues to grow around the region's schools.
Kel: "I know the names of everybody in the room."
Joe: "Well then, what is his name?"
Kel: "I don't know."
Joe: "That's a rip." *strips the back of Kel's neck*