The website "Moonhoax" (an anti-moon hoax web site that has been up and running since 1998) puts the issue this way:

"The cross hairs are called reseau-lines and were produced by a glass plate within the camera, between the lens and film.   They cause a black cross on the film where they block the light from reaching the film directly below them.   If, however, you are taking a photograph of a really bright white object, the white, over-exposed part of the film 'bleeds' into other parts of the film.  This is particularly the case if the adjacent part of the film is black.  This is what is happening where the thin reseau-lines meet a bright, reflective part of the photograph and is not unusual.  It happens on photographs with reseau-lines on Earth too.

It occurs in a number of the Apollo photographs, but you only see it where the reseau-lines seem to disappear behind a bright white part.   You never see it happening anywhere else."^[1] 

In other words, in photography, a very bright white color (from an object behind the crosshair) makes the black object (the crosshair) invisible due to saturation effects in the film emulsion. The film particles that ought to have been black were exposed by light from the adjacent brightly lit particles.^[1]  Ironically, this saturation effect would not happen if the crosshairs were drawn on in post, and so is evidence of genuine photos. Attempting to alter photos that already have crosshairs would make the compositing process far more difficult. [2]

Sources:

1. http://www.redzero.demon.co.uk/moonhoax/Cross_Hairs.htm
2. http://www.clavius.org/photoret.html

Examination of Apollo moon photos. (2009, February 27). In Wikipedia, The Free Encyclopedia. Retrieved 09:37, March 6, 2009, from http://en.wikipedia.org/w/index.php?title=Examination_of_Apollo_moon_photos&oldid=273700624