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Chocolate can seize, that is thicken or harden during melting. In recipes that contain no liquid, take great care not to let any moisture into the chocolate. In recipes that do contain liquids such as melted butter, liqueur, or water, always melt the chocolate along with these ingredients to keep its sugar particles sufficiently wet; melt a minimum of 1 tablespoon of water or liquid from the recipe with every 2 ounces of chocolate as part of the rules.
If it happens during melting, you CAN try and fix it by immediately by whisking in a little BOILING WATER (1 teaspoon at a time), and stir vigorously after each addition until the chocolate is smooth. In Beckett's book (The Science of Chocolate), he writes that about 20% by weight water vs. chocolate is needed to fix it. Note that this is total amount of water; if cream, butter or some other water-containing ingredient is used, this contribution counts. (Seize - Why this works.)
But once water is added during the melting process, the chocolate is no longer considered pure, rendering it useless for chocolate tempering or for use as pure chocolate. However, it is perfectly usable for other purposes such as chocolate sauce, ganache, drinking cocoa etc. It can also be used in a baking recipe if it isn't burned. When chocolate has been melted, it needs to be added to the recipe in a certain way, called tempering, not to be confused with the technique of chocolate tempering.