Current I is plotted against potential difference V for an ohmic conductor. The best fit line is straight, but it cuts the I axis at a positive current when V is zero. Which feature of the graph shows that a systematic error is present?

For an ohmic conductor I = V/R, so no current should flow when there is no potential difference and the line must pass through the origin. An intercept means every current reading is displaced by the same fixed amount in the same direction, which is the signature of a systematic error such as a zero error on the ammeter. Scatter about the line is random error, and the steady rise is simply Ohm's law being obeyed.