My previous broodmare had an extremely offset cannon (think this is what you are talking about) where instead of coming from the middle of the knee, it is to one side. The hoof tends to go pigeon toed to compensate for this.
Anyway, I wanted to breed from her and besides the leg she was beautifuly built. So research was done and I found that none of her parents/grandparents/siblings etc had this problem so we decided that it was a birth defect.
She bred a stunning filly with very correct confirmation.
On the competition side of things, if the cannon is crooked, I would tend to steer away from jumping as it would put stress on a 'weak' leg. Just my thoughts :)