Chocolate Seizures
What to do when you are melting chocolate and it suddenly clutches, tightens, stiffens, and hardens?
Try putting a little oil on the problem - about 1 tsp of oil per ounce of chocolate. Then beat vigorously until the chocolate agrees to behave.
Of course the best bet is to not let this happen - and most modern chefs avoid it by using the microwave to melt chocolate - not the more traditional double boiler or bain marie.
Another tip - the more additivies a chocolate has - the greater the probability it will mis-behave. Stick to really pure chocolates for the best results.