When you turn using the Cinematic Camera and disable it while turning, once you enable it again, the momentum will hold and you will start turning in the direction you were last turning.

What I expected to happen was:
I wouldn't turn

What actually happened was:
I turned in the direction that I last turned.

Steps to Reproduce:
1. Set the Cinematic Camera to a specified key in the Controls Menu
2. Press the key that you set to your Cinematic Camera.
3. Start turning, and while turning, press the key again.
4. Wait as long as you want, then press the key again, and you will begin to turn.

Additional Notes:
If you leave the world and rejoin it, you still keep the momentum.
You also keep the momentum if you leave the world, and go in a different one. Same with servers.
I did some extensive testing, and this has been a bug since Minecraft 1.3 Beta (When the Cinematic Camera was first added)

Steps to Prevent:
1. Turn with Cinematic Camera enabled (See steps 1-2 in "Steps to Reproduce")
2. Press key while turning to disable Cinematic Camera.
3. Close out of Minecraft and re-open it.
4. Enter the world and press the Cinematic Camera Key,
5. You won't turn