I've done a bit of digging myself, and I found that this bug started back in 1.8.2 prerelease 7. What happens is if you place a block of TNT on top of a wall, fence, or fence gate, and detonate the TNT (with air above it), it will phase into the block (newer versions show that, older ones have a visual glitch showing the TNT still on top on the wall/fence/fence gate, but not before starting to glitch into the block). Due to the TNT being physically within a block, it does not have as much of an impact as if it were detonated on a regular block. I assume this bug was a side affect of fixing MC-66090, the bug being that TNT would collide with a ceiling block and disregard the hightened hitbox on the walls/fences/fence gates