How to fix a dish that has gone wrong
Most kitchen disasters are recoverable, and the recoveries are not folklore. They work because of what is chemically happening, and knowing which is which saves a dinner roughly once a month.
One rule before any of them: taste before you serve, every time, while there is still time to act. Most dishes that arrive at the table wrong were fixable ten minutes earlier.
Too salty
Dilution is the only genuine fix. Add more of the unsalted body of the dish: more water and lentils to a dal, more tomatoes to a curry, more rice to a pulao.
Acid and sweetness mask saltiness without removing it, and they work well enough for mild cases. A squeeze of lemon and a pinch of sugar will make a slightly over-salted curry taste correct.
The potato trick is a myth. A raw potato dropped in absorbs a little salted liquid and takes out a negligible amount of salt. It does no harm and it will not save you.
For a dry dish there is often no fix, so serve it with something bland and generous: plain rice, plain roti, thick curd.
Too spicy
Fat and dairy actually work here, because capsaicin dissolves in fat and not in water. Cream, curd, coconut milk, butter or a handful of ground cashews will genuinely reduce the heat rather than mask it.
Sweetness helps too: a spoon of sugar or jaggery. Acid distracts from heat without lowering it.
Drinking water afterwards does nothing at all. Curd, milk or something fatty does.
Too thin, or too thick
For thin gravies, the best fix uses what is already there: mash some of the solids against the side of the pot and stir them back in. Beans, lentils, potatoes and onions all release starch when crushed.
Failing that, simmer uncovered and let water leave. This concentrates flavour as well as thickening, so it is the better option when you have time.
Cornflour slurry works and tastes slightly of nothing, so use it last. Besan works in Indian gravies but must cook for several minutes or it tastes raw.
For thick, add hot water, never cold. Cold water tightens starch and dulls the flavour.
Split, curdled or grainy
Curd and cream split when they get too hot too fast. Prevention is the real answer: take the pan off the heat, whisk the dairy with a spoonful of besan or cornflour, stir it in, and only then return to a low flame.
Once split, blend it. A stick blender will usually bring a broken gravy back together, and nobody at the table will know.
Split ganache or melted chocolate that has seized can often be rescued with a spoon of hot water or hot cream, whisked hard from the centre outwards.
Burnt
Do not stir. Stirring drags the burnt layer through everything and turns a partial loss into a total one.
Tip the unburnt food out from the top into a clean pot, leaving the base behind entirely. Taste what you rescued before adding anything to it.
If the smoky taste has carried, sweetness and acid mask it a little. If it is strongly burnt, stop. There is no recovery, and serving it teaches everyone at the table that your cooking tastes of ash.
Flat, when nothing is technically wrong
This is the most common problem and the least recognised. The dish is cooked correctly and tastes of very little.
It is almost never missing spice. It is missing salt, acid or fat, in that order of likelihood. Salt it properly, add lemon off the heat, finish with a spoon of ghee or butter, and taste after each one.
Adding more garam masala to a flat dish is the reflex, and it is usually the wrong move: you end up with something that tastes of raw spice powder and is still flat underneath.