sometimes the head of a spectator looks solid

Sometimes the head of a spectator looks solid.
I have no idea what causes this, but it appears to only happen when I'm close to a lot of redstone.

map: http://www.mediafire.com/download/dj1u67ev1npy79o/MC-93377.zip
reminder, map runs on a 15w47c server, that could also effect it

Steps to reproduce (in attached world) switch gamemode before the chunks are loaded (f3+n)