An example of perceptual defense that is often pointed to is a study by Elliot McGinnies where participants saw many flashing words, but underreported the amount of offensive words. Perceptual defense is a theory that people can protect themselves from perceiving an outside stimuli that is negative.
While there is some questionable evidence that points to perceptual defense, there is no solid proof of it. The most likely explanation of the McGinnies trial results were that participants didn't want to offend anyone else by repeated the "negative" words. In order for someone to not perceive something negative, they would first have to perceive it and decide it is negative, and then not perceive it, which is impossible.