On many calculators, graphs of rational functions produce lines at vertical asymptotes. For example, graphing y = (x – 1)/(x + 1) on the window [–4.9, 4.9] by [–4.9, 4.9] produces such a line at x = –1 on the TI-84 Plus and TI-89. But with the window [–4.7, 4.7] by  [–4.7, 4.7] on a TI-84 Plus; or [–7.9, 7.9] by [–7.9, 7.9] on a TI-89, the spurious line does not appear. Experiment with this function on your calculator, trying different windows, and try to figure out an explanation for this phenomenon.