A contract should present an interface that includes a declaration of its behavior. The declared behavior should be well defined, and if a bug in the implementation is discovered, the contract should be updatable to fix the bug. There could even be futures expressing the probability that a contract will be found to have a bug.
The willingness of participants to use a contract would depend on the chances that something about the interface is bug-prone, untested, etc.
As long as bugs do not result in reversal of money flow, the incentives seem to align properly toward a well-defined approach for declarative contracts.