If you do something without planning or meaning to do it, it's unintended. An unintended consequence of your fun day at the beach is your painful, peeling sunburn.
To intend is to plan or aim to do something. When you don't intend to do something but it happens anyway, it's unintended. An unintended side effect of your winning the 50-yard dash might be the extreme envy of your sister, who's very athletic and loves to win. And if you let her trim your hair later, you may just end up with unintended bangs!