Apophenia, commonly known in paranormal studies as "matrixing," is a common phenomena in which the brain perceives a stimuli, such as an image or a sound, as something it's not. The human brain naturally attempts to find logic in the illogical, and will detect patterns, sounds, or smells that don't exist. Examples of this include seeing the Virgin Mary in a grilled cheese sandwich, or, in terms of ghostly activity, seeing a shape of a person or face in mist, a window, or some other object.
If, for example, you take a photo of a house and notice a face in a window that wasn't there originally, it is most likely an example of apophenia. As both a paranormal investigator and as an observer, you must always be careful not to assume that what you are seeing is actual. Think of all the other examples that the media has covered over the years - food that resembles religious figures, faces in rock or tree bark - and realize that just because it appears to look like a face or figure, doesn't mean it is.
Unfortunately, if you wish to be a scientific paranormal investigator, you will often find yourself throwing away what, to the untrained eye, may seem to be excellent evidence. Apophenia is a real psychological occurrence. If you presented evidence that included examples of "matrixing" to a room of skeptics or scientists, your evidence would be quickly discounted altogether.
But how can you tell the difference between true paranormal activity and matrixing? The best advice would be to use logic and try to disprove what evidence you have. Have you captured what appears to be a mist-like figure in a photo? Ask yourself: was it cold that night, and could it be someone's breath? Was someone nearby smoking? What other forms of contamination could it be? If there's even the slightest chance it could be human contamination, then it's not a credible piece of evidence. If it appears to be a face or figure in a reflective surface, think about the weather conditions. It could very well be the clouds in the sky reflecting in such a way as to appear as something recognizable. Which, if you think about it, makes total sense - everyone at some point has "seen" something in the clouds.
So take caution in what you're seeing and the evidence you collect. Never jump to conclusions without thinking about all the different ways what you think you're seeing could be something else. Apophenia is a common occurrence for everyone, and can easily lead to assumptions that simply aren't true.