Quis custodiet ipsos custodes? Apparently, Nobody.

"Guards 1, 2, and 3. Guard 1 watches the treasure. Guard 2 watches guard 1. Guard 3 watches guard 2. But then guard 1 also watches guard 3. If  “watching” implies being successful in creating truthful behavior, then the “circle” represented by such a system is successful. In this circle any guard can also be an intervenor."

Leonid Hurwicz