I have one client who had a severe mental meltdown brought on by extreme stress, fatigue, anxiety, and working more than what they human body can handle. When this meltdown occured, some of his anxious worries manifested, in his mind, as an image of god showing him what will happen in the future (things that are happening, such as pharmaceuticals ending up in the water supply after they have passed through us or we flush them), and he took a picture that he believes reveals the image of Jesus. From his perspective, he isn't lying and treatment will revolve around him focusing less on the doomsday visions. It doesn't reflect a willingness to lie, however.
Sometimes there is no advantage to a lie, but yet sometimes people still do it (kids can especially fall in this category).