有意栽花花不发，无心插柳柳成阴
yǒu yì zāi huā huā bù fā , wú xīn chā liǔ liǔ chéng yīn
lit. you plant a garden and the flowers do not bloom, you poke a stick in the mud and it grows into a tree
fig. things do not always turn out as one would expect; well-laid plans may fail, and success may come where you least expect it