IT'S MAGIC!!!...Well, not really...

In the mosaic world, it's now all the rage to do "morphing mosaics" where when you're looking at a mosaic from one angle, it appears to be just a single image, but as you shift to another side, the image morphs into another image.

This effect was first done with Chris Doyle's Portrait of Dorian Bley using "cheese slopes" put next to each other. Looking at the image strait on will make it look really weird, so using this, you get some very cool transforming creations. Now, it's been copied and re-done all across the community and I only suspect it to gain more traction:

Arthur Gugick's Darth Vader/Anakin mosaic.

Daniel Stoeffler's Lego Harry Potter/Lord Voldemort (he who must not be named!) mosaic.

Reasonably Clever Chris's original Dorian Bley featuring the original technique.