Everyone knows someone who believes in karma or even yourself, but is it real? The principle of karma is that every act done by a person will return to that person eventually: a good act will be returned with good; an evil act with evil. In Buddhist traditions, this cycle is treated as a natural law of cause and effect, whereas in theistic Hindu traditions, God is responsible for ensuring karmic rewards and punishments. Since bad things often happen to good people, and vice versa, it is important that the return on good and evil actions is not direct, but can happen much later. For this reason, belief in karma is linked to belief in reincarnation, when an apparently good person endures a life of misfortune, it is the deserved result of evils in a past life. Similarly, when a person apparently gets away with many evil actions throughout their life, karma ensures that this will return to cause them misery in a future life.
There is no plausible evidence for the existence of reincarnation, without which the principle that good deeds are always rewarded and evil always punished is obviously false because experience provides many counter examples. When combined with the theory of reincarnation, the principle of karma return becomes an entirely untestable supposition. Since we can never know the life history of the entire sequence of past and future selves of an individual, it is impossible to evaluate whether or not the good deeds they have done have been met with equally good consequences. Karma becomes an unfalsifiable principle as soon as it is admitted that the balance of good and evil need not be restored in a single lifetime. Within most interpretations of the concept of karma, it's untestability means it is impossible to refute or support through evidence. So although everyone at some point believes they have witnessed or had karma, there is no actual proof that it is real.