In experiments like this, the sources of error include:
– the holes created by the pins are too big, hence the line drawn using the two holes may not be very precise, hence affecting the distance/angle measured (depends on experiments)
– When putting back the glass block back on the paper, the position may be not exactly the same, hence affecting the angle/distance measured.
As a precaution, it is good to ensure that the distance between the two pins are more than 4 – 5 cm. With the holes further apart, the line drawn will be more accurate.