Taste before serving and adjust salt and acid — the last 30 seconds of seasoning matter more than any single ingredient.
The sauce broke into oily puddles
Too much fat too fast, or too much heat. Whisk a tablespoon of warm water in a clean bowl and beat the broken sauce into it a spoonful at a time — you're starting a new emulsion and feeding it the old one.
It's too thick
Thickness means crowded, not broken — loosen with warm water a teaspoon at a time.
Does the alcohol really cook off?
Mostly. A hard 2–3 minute boil drives off the harsh raw alcohol; long simmering removes more. A splash of flavor compounds carried by the wine stays behind — that's the point.
Which wine should I use?
One you would drink, but nothing precious. For reds: dry, mid-weight, low-oak (Côtes du Rhône energy). For whites: dry and crisp (Sauvignon Blanc, unoaked Chardonnay). Sweet or heavily oaked wines concentrate into syrup or bitterness.
How do I know when it's reduced enough?
Drag a spoon across the pan: the sauce should hold a brief trail. Nappe — coating the back of a spoon — is the classic test. It thickens further as it cools, so stop a shade early.