2A03 has a bug that resets the duty phase when switching between certain frequencies. This can be heard as a pop and might be noticable when using vibrato or pitch slides across those frequencies. There are eight groups of frequency registers where sliding the frequency register from one group to another resets the duty phase: 7FF-700, 6FF-600, 5FF-500, 4FF-400, 3FF-300, 2FF-200, 1FF-100, FF-0.