In Disney's Sleeping Beauty, Maleficent curses the newborn Aurora: before the sun sets on her 16th birthday, the girl will prick her finger on a spinning wheel and die.
But, for some reason, having issued the curse, Maleficent turns out to lack all confidence in its power. She apparently spends the next 15 years trying to find Aurora after the good fairies hide her away in the forest, and then, having done so on the eve of the girl's 16th birthday, makes strenuous efforts to appear, with a spinning wheel, in order to force Aurora to touch the needle.
This appears not to be a curse at all, in fact; it is more like a declaration of intent. (And a bizarre one at that: why doesn't she just turn up at the end and kill Aurora outright, given that she knows that Merryweather has cast a partial dispellation of the curse?)
But perhaps this is what a curse really is: not a statement of intent, per se, but a spell which binds both the cursed and the curser to perform a particular sequence of actions through to a desired end. Aurora will encounter a spinning wheel on her 16th birthday, but Maleficent must play her own role in ensuring this in fact transpires. A curse, seen in this way, is something like a willed destiny - a final destination that will come about if only the one issuing the curse makes sure that it does.
An even more accurate way of putting it is: a curse is a stated plan which will come about if the right actions are performed at the right time. Ordinary plans, no matter how perfect, go wrong because of bad luck. A curse will not (barring interference from fairy godmothers) as long as the curser holds up at his end.
This makes fairy tale curses (as opposed to "you lose half your STR for the duration of the curse"-style D&D affairs) potentially, and interestingly, gameable. Magic-users can cast curses, and specify an outcome in doing so, but they also have to specify the steps they need to take to ensure that the outcome becomes real. If they do, the curse is realised. If they fail - too bad, and perhaps additional horrible consequences which follow. Maleficent meets her end having been impaled on a thrown sword. I'm sure that more interesting possibilities could suggest themselves.