Error variances of OLS is assumed constant (homoscedasticity). However, when
error variances are not constant, there is heteroscedasticity. When there is heterscedasticity, OLS estimations places more weight on the observations with large error variances than those with smaller error variances. The OLS parameter estimators therefore are unbiased and consistent, but they are not efficient, therefore the consequence is similar to those of autocorrelation.