We propose a model-based approach to integrate human error analysis with task modelling, introducing the concept of Error Pattern. Error Patterns are prototypical deviations from abstract task models, expressed in a formal way by a model transformation. A collection of typical errors taken from the literature on human errors is described within our framework. The intent is that the human factors specialist will produce the task models taking an error-free perspective, producing small and useful task models. The specialist will then choose from the collection of error patterns, and selectively apply these patterns to parts of the original task model, thus producing a transformed model exhibiting erroneous user behaviour. This transformed task model can be used at various stages of the design process, to investigate the system’s reaction to erroneous behaviour or to generate test sequences.