A peccadillo is a minor offense or sin. Parents recognize that their kids have a few peccadilloes: they don't always remember to say please and thank you, don't put their dirty clothes in the hamper, and worst of all, they keep finding the chocolate stash!
Peccadillo is based on the Spanish word peccado, meaning "sin," with a diminutive added, making peccadillo a small sin. How big a sin is a peccadillo? Well, that depends on whom you ask. The person committing the offense is likely to try to pass off any number of transgressions and mistakes as peccadilloes. If you apologize and are forgiven, it's probably a peccadillo. If your transgression could get you fired, it's probably not!