It's not an error but a standard issue stamp. There are 50 stamps to a sheet (pane) and the triptych format (3 stamps making up one image) means that there is always an extra stamp in each row.
Since the stamps are printed in sheets of 200 and cut into panes of 50 stamps each, the panes on the left side will have the image of Scott 1631 (fifer) with the perforations running through it; the panes on the right side will have the image of Scott 1629 (drummer boy) with perforations running through it (as in the example shown in the original post).
The best image I could come up with quickly to illustrate the differences is shown below:

To better illustrate this, a full pane that includes the stamp in question looks like this:
