Problem: 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) to know that it wasn't your mistake. 

Solution: Simply place [sic] after the error. 

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