It is only by repeatedly thinking the same things (good or bad) that they manifest. So if you have one off bad thoughts don't worry and gently replace them with a positive one.

Everytime you think a negative thought gently correct yourself. Over a period of time you will start thinking only positive thoughts without even noticing.

To curse someone and for it to hold true for that person, you need immense spiritual powers. I think 99.9% of people do not fall in this category.

Instead bad thoughts we hold for someone else cause negative reactions in our own self - we loose our happiness and repeated negative thinking will manifest as bad health since the body produces harmful stress hormones.

So where ever possible think good and happy thoughts. Practice it daily with some positive meditation. Remember:

Whatever you think, that you become. Therefore if you have to think, think good thoughts, great thoughts."
-- Swami Vivekananda

"You are what your deep, driving desire is.
As is your desire, so is your will.
As is your will, so is your deed,
As is your deed, so is your destiny."
-- A verse in the Brihadaranyaka Upanishad,