When you fly a plane upside-down, your head also inverts. It is quite disconcerting.

...that looks perfectly normal to me. If you're upside-down, your head should also be upside-down.

It's more the "flipped 180 degrees and staring backwards" part that has me concerned. :P

That's because the player's head points in the direction of the player's yaw, not the direction the camera is pointing. When you do a 180 in a plane, you go past vertical pitch, which causes the yaw of the player to be inverted of where it normally would be. When you add a clockwise angle to the camera you add a clockwise yaw of the head, which is actually counter-clockwise in that orientation. I should be able to fix this with some trickery like I do the player body...

Fixed in Commit 457ae66.