Why does fondue sometimes "curdle" and what can I do to prevent it? Once when I was making a pretty standard fondue (50% gruyere, 50% emmenthal, white wine) the cheese somehow "curdled" and became a mix of a slightly fondue-tasting liquid and a rubbery ball of cheese, completely useless for fondue purposes.

Why and how does this happen? Was it the quality of the cheese, did I overheat the wine before adding the cheese, or did I add too much/too little cheese at once?
Normally, you start adding the cheese when the wine is simmering and stir regularly as you add it. If necessary, you can add a tablespoon of corn starch dissolved in some white wine to homogenize the mixture.