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