It's a well known knitting myth that knitting a sweater for a significant other before marriage can lead to the knitting curse that the relationship will break up. There is even a Ravelry group testing the curse. You can find it here. There's another group for believers of the curse here.
The belief in this curse is so pervasive there is a Wikipedia page here. That page does give some alternative explanations for the myth.
Has it happened to me you ask? Unfortunately yes! Do I believe the myth? No, but then I'm not really the superstitious type.