In agent-oriented programming and planning, agents' actions are typically specified in terms of postconditions, and the model of execution assumes that the environment carries the actions out exactly as specified. That is, it is assumed that the state of the environment after an action has been executed will satisfy its postcondition. In reality, however, such environments are rare: the actual execution of an action may always fail, and the envisaged outcome is not met. In this paper, we provide a conceptual framework for reasoning about success and failure of agents' behaviours. In particular, we propose a measure that reflects how "good" an environment is with respect to agent's capabilities and a given goal it might pursue. We also discuss which types of goals are worth pursuing, depending on the type of the environment the agent is acting in.