Once early on in my tripping career, I did 2 hits of some already pretty strong LSD for the first time. While sitting next to the doorway of totally dark room, flipping through records stacked nearby, I got a growing senseof being watched. With dread I suddenly looked to my right, into what I know was a totally black room, to see what appeared to be an african mask of a face, size of a normal human head floating mid-room, in brightly painted colors, it had eyes that were glancing back and forth and a mouth that was slightly moving, as if it were trying to speak.
My first time on Mushrooms, a strong 3.5g dose, EVERY SURFACE IN THE ROOM, reguarless of if it was flat, curved, solid color or already had a design on it was COVERED with interconnected glowing neon parallel lines that bent at the edges of outlines to fill every shape with repeated designs, these hallucinated neon tubes each and every one was filled with gleaming bright spots that were all flowing along inside these non-existent glowing tubes I would classify that as so extreme as to be called "seeing something that is not there." There was were these tall doors of a wooden clothes dresser with a faint wood grain pattern. Later, after the neon tubes had faded out, the woodgrain of these doors, as I watched morphed and shifted in a rather mesmerizing fashion from a random wood grain pattern into a single large bearded Jesus-like face, which was animated, and gazed at me knowlingly and benificently, smiling. We looked at each other for a few minutes, then without warning, it un-morphed back into the wood grain outlines. I would definitely call THAT seeing something that is not there.
So there are examples from both drugs. I've also, dont recall on which drug, but I clearly remember once on a hefty dose, along with a bunch of very strong hash and some percussive hypnotic music, I had been looking at a vase on top of a TV which was off... dim colored lights in the room... stared at it for a little bit, and watched as it melted, but then reformed from the blob of pulsating color into the form of a miniature lion or some other big cat and started pacing back and forth on top of the TV set looking over at me as it did so... My eyes got itchy from staring without blinking for so long, so I rubbed them and when I looked back, it was a vase again. I think that qualifies as "shit that isnt really there."
Not like a constant visual field that completely\ replaces reality with 100% hallucination like I read DMT can do, but definitely does happen simetimes to me.
And of course on both drugs, closing your eyes for a while, especially listening to music, seeing all manner of "stuff that is not really there" replacing what should just be the black insides of your eyeballs with anything from kaleidoscopes to animated Aztek circuit diagrams, to mountains/cities/planetscapes/alien forests.