The dissociation of a preparation

For example, here is a chocolate-hazelnut ganache that is tranché:

It doesn't show up very well in the (small) photo, but there's a thick chocolate-hazelnut pan base, and on top a layer of liquid fat.

What happened?

In the vast majority of cases, it's because in your mixture there is a fatty part and a watery part, the whole thing comes together in a delicate balance.
If this balance is broken because there is too much of one or the other, or a thermal shock, the whole which was homogeneous dissociates, it cuts!

This imbalance is often caused by the heating, perhaps excessive, of the preparation, and during this heating the water can evaporate and reduce, the fat not.
This is what happened with the ganache above, I didn't remove the simmering cream from the heat soon enough before adding the chocolate and hazelnut powder, sad result.
Another example, a bearnaise sauce or a hollandaise sauce that slices when it was almost finished, it's certainly because there was too much butter added and not enough water in the vinegar-wine-herb or lemon reduction.

And besides, it doesn't prevent this tragedy, there is little or no warning of disaster for an amateur, it is quite brutal.

What to do if it does?

1) Don't panic, don't despair, because it's very easy in this situation, so great is the disappointment, to swear a good big blow, to say to oneself that it's ruined, and to throw everything away...
2) Keep a critical eye: OK it's tranché, but what if it lacks water? Let's try to compensate it, and for that :

- Remove immediately from the heat or the water bath
- Add, very cold, a little bit of water (Béarnaise) or one of the components that brings it (cream for the ganache).
- Take the mixture back slowly, with a whisk, a spatula or a maryse to incorporate this addition (cross your fingers too, put all the chances on your side ;-).

3) If it still doesn't work, use the super power of cornstarch:

- Incorporate a mixture made of a tablespoon of water and a teaspoon of cornstarch in the preparation that you will put back on the fire and will whip until return of the initial texture.

You will have understood, there is no fatality in these small culinary dramas, it is just a bit of physics in your pan, it is necessary to try to be attentive enough during your preparation, to prevent rather than to cure.

To sum up: A preparation that goes off is most of the time an imbalance of water/fat, an imbalance that you can try to reduce or eliminate to regain the initial smoothness.