有意栽花花不发,无心插柳柳成阴 yǒuyìzāihuāhuābùfāwúxīnchāliǔliǔchéngyīn plans often fail but luck can bring success
Literally "to plant flowers with intent and have them not bloom, while sticking a willow branch in the ground without thought and having it grow into shade"; describes how well-laid plans may fail while success can come unexpectedly from casual actions.