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In recent research on the optimization of variable fractional delay (VFD) filters, group delay errors have been paid additional attention besides the conventional criterion of frequency response errors. While group delay physically measures the delay of the magnitude envelop of a signal, an important application of the VFD filters is to estimate the instantaneous delayed samples, such as in the applications of interpolation. In view of this, the impact of the group delay error on the estimation of instantaneous samples is investigated in this brief. Analysis and simulation results show that in such applications, the traditional criterion of frequency response errors reflects the time-domain errors and group delay errors do not show a slight impact on the time-domain errors.