When on the slowest speed in spectator mode, if you stand still, look slightly diagonal (specifically up to 42.3 degrees off axis) and press W, you go along the closest (horizontal) axis rather than the direction you're looking.

What I expected to happen was...:
I would walk in the direction I was looking

What actually happened was...:
I walked along the X or Z axis 

Steps to Reproduce:
1. In spectator mode, stand still.
2. Using the scroll wheel, set your movement speed as slow as possible while still being able to move.
3. Look slightly off axis (+/- 42.3 degrees) and walk forward, and you'll notice that you aren't walking straight.

Something to Note:

If you are already moving and slow your movement speed down to the slowest, you'll continue moving in the direction you're looking. The 42.3 degrees that I mention in this is based on anecdotal research. I don't have any reason as to why its that, all i know is that if its 42.4 degrees then you'll move as normal.