Rarity spent HOURS brushing Pinkie's mane, trying to get it straight. When she finally succeeded, Pinkie became so excited that she decided to throw a party! The thought of the party caused her mane to instantly poof back up again to its usual curly state.
Rarity swore never to brush Pinkie's mane again.
Then she had fun at Pinkie's My Mane Was Finally Straight For Three Full Seconds party and promptly forgot that she ever swore anything. (She probably would not have been able to say no if Pinkie had asked her again anyway.)