What I expected to happen was for there to be no interaction whatsoever with the world while in spectator mode.

However, as I flew through the ground in spectator mode (at sprint speed), I looked down and saw the sprint particles of whatever block I was flying through.

To reproduce:
1) Enter spectator mode.
2) Go into the ground.
3) Fly forward and press your "sprint" key.
4) Look down. You will see the "running" particles of whatever block you are currently going through.