Sometimes the boiling sugar and water mixture appears to break just before it reaches a temperature stage. If this happens, take it off the heat immediately and stir it gently. You should be able to get it to recombine. Use a medium-high heat so the syrup cooks relatively quickly.
Also, be sure you're using high quality butter - never margarines, which may contain too much water for candy making.
If you are in a very humid climate or a rainy one, choose drier days for candy-making. Excess water - which can even come from the air, can also cause butter/sugar syrups to break.
See http://baking911.com/learn/baked-goods/candy/problems-and-solutions
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See http://baking911.com/learn/baked-goods/candy/problems-and-solutions