The most common concerns are a result of the font being monospace. In most proportional fonts, 1 has a hat, and l is a vertical line; and 0 is a thin, squared

The most common concerns are a result of the font being monospace. In most proportional fonts, 1 has a hat, and l is a vertical line; and 0 is a thin, squared off oval; while O is almost a circle. The characters become similar in monospaced typefaces so that they can fill the rectangle they're supposed to fill. I and l are often indistinguishable; but there's no shortage of fonts that put the extra strokes onto I [eg: Verdana, Tahoma, every serif font...]