Confounding
Confounding is an extreme case of a lurking variable that is perfectly correlated with an experimental factor. The effect of a confounded factor cannot be estimated so it is particularly important that confounding is avoided when data are collected.
Experiment with a new variety of wheat
The experiment was a waste of time and money — no conclusions can be reached about the new variety.
Trial of a new teaching method
The data do not allow you to reach any conclusions about whether the resource is effective.