If he is sound and doesn't appear in pain, no issues. I had a horse who was quite young, a tb who was over at the knee. I started jumping him shortly after buying him and he very quickly decline. He became very lame and unsound and nothing could be done. I ended up selling him to someone who used him as a flatting/little girl horse. Worked out well
One of my trainer's grand prix horses is slightly over at the knee and he has no problems. One of our well known vets (here locally) has a really good jumper who used to do grand prix courses as well (now retired) and he is over at the knee and he's never had problems.
It depends on the horse. Sometimes its just a conformation fault that never causes problems, sometimes its a very serious issue which will really prevent you using the horse to its full potential.