I found this weird illusion with item frames while I was inside my friend's house, so I decided to make a wall myself and see if it showed up again. Turns out it did, and I wanted to show you guys here on PMC. If I'm not mistaken, it should appear that the item frames on the dark clay are bigger than the light clay, whereas they're actually both the same size (Check it in Singleplayer if you don't believe me).

Note: Made on pvp.mineowner.com