Although not as much of an issue in a short poem as it is in an epic poem, novel, or other long work, it is always important to have your details straight. In a short piece, there are fewer details to worry about. Map out your poem. What time of day is it? Are there characters? Who are they? What year were they born? Any detail that may come into play multiple times during the poem or song should stay consistent. If the character is fifteen years old in 1830, he’d better be 35 in 1850. If the character’s hat is red in the beginning of the poem, unless you state or at least imply how it changed color, the hat had better be red at the end. If the details are not consistent, it brings a jarring note to the poem as the reader hears or reads it. Every rule has exceptions, and sometimes, especially in humorous works, a certain silly slip of the details is warranted.