Problems with model specification, such as missing variables/predictors, wrong functional form, wrong variance or covariance structure, etc.

Misspecification of statistical models such as missing variables/predictors, wrong functional form, wrong specification of variance or covariance structure or other problems with model specification.

See https://en.wikipedia.org/wiki/Specification_(regression) and http://www.statisticshowto.com/model-misspecification/