Hitting a mob or object while sprinting breaks your sprinting streak.

Further explanation
While you are sprinting, successfully hit any mob or player make you walk again.
Why is this a bug?
On PC, you will not slow down (Ctrl + Forward) when hit a player while sprinting.
Have not tried Forward + Forward, if the result is different from Ctrl + Forward, this may also be PC sprinting exploit. (Sprinting never breaks)
This could however be wrong. Check and test: http://minecraft.gamepedia.com/Sprinting
Step to reproduce
1. Start sprinting and hold your sprint.
2. Hit a mob or player.
3. Check out if you are still sprinting.

Confirmed on my iPad Mini tablet. 0.12.1 Full Release