One reason for using a feedback amp is to develop a closed loop gain that is independent of op amp characteristics. When bandwidth and slew rate are looked at, loop gain should also be considered. Gain error will be a problem at higher frequencies. Errors in the closed loop gain depend on the ratio between the open loop gain and the closed loop gain. So, as the signal frequency increases, the amount of gain will decrease as the closed loop gain gets closer to the open loop gain which results in increased gain error. This error causes distortion because it affects higher frequency signals more than lower frequency signals.