In some instances, your source may have an error, such as bad grammar, a misspelled word, or a historical error.  You want to quote your source exactly, but you also want your reader (and teacher, who will be grading the paper) that it wasn't your mistake.  Simply place [sic] after the error. 

"Investigators were never sure who done it [sic], but the killer left several clues" (Bubba).