Don't worry about it people do it all the time on purpose its called super cropping it will slow down the growth on that branch till it heals but it will be stronger in the end. I used to do it when I would grow 4 plants under 1800 watts so the plants could hold all the weight. < I would get 3 lbs off the 4 plants. Here is a run down of the technique that I use to supercrop there are also other ways.< It will slow down bud growth if you do it flowering but I wouldn't worry to much about the one you did accidentally.
Crimping
Crimping is an advanced technique whereby you break the inner herd of the plant without damaging the outer, which is where all the plants strength comes from. Break open the stalk of a cannabis plant and you'll notice immediately the hard, outer herd. This is where textiles come from. The soft inner, lighter colored material is pure cellulose - the phloem and xylem.
In vascular plants, phloem is the living tissue that carries organic nutrients, particularly sucrose, a sugar, to all parts of the plant where needed. In trees, the phloem is the innermost layer of the bark, hence the name, derived from the Greek word φλοιος (phloios) for "bark". The phloem is mainly concerned with the transport of glucose and starch made during photosynthesis.
The plant passes water and nutrients cell to cell through this network of cellulose. Breaking the plant's inner walls will cause it to rebuild. But it rebuilds these networks better than they were before; it rebuilds so fast (under good growth conditions) that 24 hours later the plant is using the new highways and it's increased capacity for moving water and nutrients.
This technique can begin as early as the seedling stage. Twisting the plant gently, using both hands so you don't pull on it's roots. Work each of the plants branches every 2", up to once week. Do this by grasping the spot to be treated with two fingers of each hand. Use one hand to stabilize, while the other gently, slowly twists. Or you can twist with both hands in opposite directions, slowly until you feel a slight snap. When the plant's get heartier you will actually be able to hear it.
On smaller branches, you do not need to twist, as firm finger pressure will rupture the herd.
You can do this from a seedling all the way up to right before you flower. You will hear a snap or in younger growth you will feel it give under pressure of a firm pinch, and when it heals you will get a knuckle there
I do this instead of topping and then tie them down it really does make for really strong branches
Heres a couple youtube vids of a different style of super cropping
http://www.youtube.com/watch?v=H1YEc7ENd5E
http://www.youtube.com/watch?v=2FmXHhKFA5U&NR=1