Good control conditions, like fine food and wine, are easy to admire and hard to produce. It helps to be aware of some common pitfalls: four effects are particularly relevant to experimental AI. They are called ceiling, floor, regression, and order effects, respectively, and spurious effects, collectively. Spurious effects suggest that a treatment is effective when it isn't, or suggest that a treatment isn't effective when it is. Ceiling and floor effects are sometimes easy to see, regression and order effects can be quite subtle.