What probably happened
Your emulsion separated. The fat and water in your sauce stopped holding hands and went their separate ways.
Why it happened
A sauce like hollandaise, vinaigrette, or pan sauce is an emulsion — tiny droplets of fat suspended in water (or vice versa), held together by an emulsifier like egg yolk, mustard, or butter solids. Emulsions break when: (1) you add fat too fast and the droplets can't disperse, (2) the temperature gets too high and the emulsifier denatures, or (3) the sauce gets too cold and the fat solidifies and separates. Temperature is the most common culprit — butter-based sauces break around 160°F when the butter solids separate.
How to save it now
For a broken hollandaise or butter sauce: put a teaspoon of cold water in a clean bowl and slowly whisk the broken sauce into it, drop by drop at first, then in a thin stream. The cold water helps re-emulsify the fat. For a broken vinaigrette: whisk in a small spoonful of mustard or honey — both are powerful emulsifiers that can rebind a separated dressing.
How to prevent it next time
Add fat slowly, especially at the start. Whisk constantly. Keep the temperature moderate — if the bowl feels hot to the touch, pull it off the heat for a moment. Have a tablespoon of cold water ready as insurance. The moment the sauce looks oily or grainy, whisk in the cold water immediately.
Tiny kitchen test
Make a simple vinaigrette: 3 parts oil to 1 part vinegar. Whisk the oil in slowly and it'll stay creamy. Dump all the oil in at once and shake — it'll separate within 30 seconds. Same ingredients, different technique.
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