Usually called ringing. it happens right after sharp turns like the 90 degree turn from the letter recess out to the face of the part. Due to the Prusa's design not having a huge amount of rigidity in the Y direction. Lower your acceleration rates to help with this. printbus did a great write up of the several places you can set and change acceleration values. 2x in firmware and again in the slicing program IIRC. Also, you can make a small improvement by orienting the parts so the face with the worst ringing issues/best appearance needed is parallel to the X axis.