Since this qualifies as an exploit, I'm guessing that this report may well be a duplicate of some private ticket that I can't search for, but I'll post it anyway:

My test world has a small cave opening, through which you can consistently x-ray if in an adjacent chunk. I have not been able to reproduce this in any other world, and I don't know what the specific conditions might be that cause this bug to occur.

The world seed is 'mojira'. Coordinates shown in screenshot, alongside a picture of what the cave should look like. I have also attached a screenshot of my video settings, in case that happens to be useful.