The problem is even old software can sometimes not be trusted. OpenSSL?
In some cases intelligence agencies put backdoors (bugs) strategically in certain software.

Turing complete smart contracts will never be fully trusted but as Dan says, the best we can do is use the most established code which has been checked the most. At the same time the purpose of smart contracts, blockchains, or code is to serve communities. These communities may include participants that are machines and humans but in the end a community requires some kind of governance to determine what to do if code fails or in order to always make sure the best interests of the community are considered.