I expected that since I am in Spectator mode, I can sprint through blocks unnoticed. But,

When I sprinted through blocks I left behind sprinting particles and it gave away my location a bit to my friend (We were playing Spectator hide and seek)

To recreate:

1. Go in Spectator mode
2. Go underground (Into blocks) and start sprint-flying (double-tap Forward or Ctrl key)
3. You should notice that you leave behind the sprinting particles you normally shouldn't in SP mode.

Just a small bug, but it is annoying.