Brain buster for you astronomy buffs?

After watching an episode of shanedk's excellent video series, Bogosity, debunking the Apollo landing hoax, I stepped outside to just look at the Moon for a bit. I saw a halo around the Moon, but it was not circular - it was rectangular, and furthermore, the Moon was not centered in this grayish rectangle, but located in the lower-left region of it.

It only took me a few seconds to realize what caused this, but I thought it might be fun to see who else out there can guess the cause. I thought of this because it reminded me of the sometimes strange claims that are posted here. I know that a real Moon halo would be circular, obviously, but if I read a claim of a square halo (which disappeared after a minute or two), I would be skeptical.

Can you debunk this phenomenon?