I spent a good 10 minutes trying to find if this was a duplicate and found nothing on MCPE only MCPC. I really hope this isn’t a dupe because I spent like an hour looking at almost every block.
What happens: Glass, Stained Glass, Slime Blocks, and other transparent or semi-transparent blocks can see through some other blocks and all mobs. This includes many of the blocks in the picture below aswell as many more like those blocks. It can also see through all mobs as shown by it seeing through a Zombie, Skeleton, and a Boat.
How to do: Place any one of the blocks below. Drop glass or another transparent block between you and the item. Look through the transparent block and see through the other.
Attachment explanation: One image shows most of the blocks that I found this glitch occurs with. Some blocks are not there because they are simmilar to ones already present like Iron Trapdoors and the many different Rails. Another image shows me seeing through many of the blocks there with glass. The final image is of me looking through the glass to show it going through entities like mobs and boats.