In 1.7.10 and earlier, TNT will immediately collide with the ceiling and just fall straight down if it's triggered when there's a block above it. This no longer happens and it ignores ceiling collision when triggered.
Example gfy: http://gfycat.com/LimitedAppropriateAmurminnow
Edit: Bonus info, taken from comment below by Dan Shemp
PrimedTNT now has its "centre" at the bottom of the entity, which means the block code for TNT summons the entity 0.5 of a block too high. When "+ 0.5f" is removed from the y-argument for PrimedTNT's constructor call, this bug is resolved. Dispensers also had this problem but it was fixed in separate code.
Somewhat related: Ender Crystal entity spawning 1 block too low
Linked issues
Comments 15
Perhaps some of the code dealing with the TNT affects both this ticket and MC-64025, but this ticket is definitely not a direct duplicate. At the very most, it Relates (and that's stretching it).
This ticket simply describes the lack of TNT ceiling collision. I tested, and it is indeed present in 14w32d, but absent in 1.7.10
Dupe of MC-8565.
Dupe of MC-8565.
Nope. That one is about dispensing TNT directly into a block.
This is new with 1.8, it has to do with the "hop" when you trigger a tnt block not factoring in ceiling collision.
PrimedTNT now has its "centre" at the bottom of the entity, which means the block code for TNT summons the entity 0.5 of a block too high. When "+ 0.5f" is removed from the y-argument for PrimedTNT's constructor call, this bug is resolved. Dispensers also had this problem but it was fixed in separate code.
Somewhat related: Ender Crystal entity spawning 1 block too low
michael
Do you just not read reports anymore or do you just guess what it says from their keywords. This was immediately marked as a duplicate of a bug (MC-64025) that the only similar word between them is "TNT". This has to do with tnt not colliding with a ceiling when activated, and MC-64025 has to do with a visual glitch when tnt is dispensed inside of another block.