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Now and then someone washes a plant with neem and finds brown, scorched patches on the leaves a day later. It is nearly always one of four things, and none of them is a fault in the oil.
Sun on wet leaves
Neem oil is a thick vegetable oil. Misted on a leaf it forms a thin film, and a film of oil in direct sun heats the leaf surface under it. Wash in the morning or evening, and keep the plant out of strong light until the leaves are dry.
Too strong a mix
A leaf wash is a weak mix: about 5 ml of oil in a litre of water, carried in with a little soap. Neat oil, or a mix made by eye and far too rich, sits on the leaf as a greasy layer and marks it. If in doubt, make it weaker. Rates are on the leaf-shine spray page.
Heat
A conservatory at 30 °C is the wrong place to wash a plant. Oil on a hot leaf does damage that the same oil on a cool leaf does not. Wait for a cool morning.
The wrong plant
Thin, soft, fuzzy or very young leaves mark easily whatever you put on them. Ferns, African violets, seedlings and fresh spring growth are better left alone or dusted with a brush. Thick, glossy leaves, the ones a leaf wash is for, shrug it off.
Do one leaf first
On any plant you have not washed before, mist a single leaf, wait a day, and look. If it is fine, do the rest. If it has marked, you have lost one leaf rather than a plant, and the answer is usually a weaker mix or a cooler time of day.