In yet another perfect illustration of the colossal stupidity of modern drug enforcement, here's the story of a drug transaction in which both parties turned out to be police:

An undercover Iredell County Sheriff's Office deputy recently purchased drugs from undercover Statesville police officers, raising questions about communications between the two agencies.

Statesville Police Chief Tom Anderson said undercover officers from his department were working a week-long case when they met with someone interested in selling a small amount of marijuana.
…
After the arrest, investigators from the sheriff's office arrived and confirmed the seller was an undercover deputy and he was released, Anderson said. [Statesville Record & Landmark]

Pete Guither gets it right:

Good thing they were able to stop that small amount of marijuana they were selling from reaching the streets.

Seriously, this is what happens when you have police posing as perps at every level of the drug business. Drug enforcement is all about creating crimes that would never otherwise have occurred, and there are about a million ways that it can go wrong. As funny as this story is, the harsh reality is that frequently when police sell drugs, it's not part of a planned operation. It's because they are actually just straight-up selling drugs.