A measurement error occurs when some attribute values are incorrect or inaccurate. Note that measurement of physical properties by continuous values is always subject to some error.
In supervised learning, classification error means that a training example has an incorrect class label.
In addition to errors, training examples may have missing attribute values. That is, the values of some attribute values are not recorded.
Noisy data can cause learning algorithms to fail to converge to a concept description or to build a concept description that has poor classification accuracy on unseen examples. This is often due to over fitting.
For methods to minimize the effects of noise, see Overfitting.