As Victor said, it is a wider range of circumstances where this happens. Please update this to be slightly more generalized as TNT in water sometimes (most of the time for me) blows up blocks around it, not just when dispensed from an upwards dispenser with a block on top.
Lots of things that have been marked here as duplicates, where in actual fact this should have been marked under one of them as this only explains "part" of the bug.
Confirmed on 14w05a & 14w05b
As Freddy said, in 14w05b everything works "normally", but in 14w06b things get blown up.
As Victor said, it is a wider range of circumstances where this happens. Please update this to be slightly more generalized as TNT in water sometimes (most of the time for me) blows up blocks around it, not just when dispensed from an upwards dispenser with a block on top.
Lots of things that have been marked here as duplicates, where in actual fact this should have been marked under one of them as this only explains "part" of the bug.