A few times each year, this first quarter moon phenomenon happens to coincide with the moon facing North America. It is a fun, although temporary, experience. It is only visible for around 4 hours, after which light floods thru. Until then, small slits of light cascade over a pair of craters, forming the "x".

The picture below was taken afocally, hand-held at the eyepiece. The "X" appears just barely on the "Dark" side, about 1/3rd of the way in from the right.