Here's another classic: the its/it's conundrum. As found in a section heading:

It is the "magic hour" for energy-independent buildings like the Malpica, that time of day when it's photovoltaic windows capture the setting sun with their almost imperceptible magenta hue.

While this is a great sentence, full of wonderful imagery, Brooks uses a contraction (it is) where instead he wants a possessive.

Source: World War Z, p. 228