Why do many minifigures have printing on their neck pegs?
It is explained and demonstrated in this documentary from the LEGO factory: https://www.youtube.com/watch?v=wnRRDIFNxoM&amp;t=3m33s

While the machines in Denmark use a sensor to detect the paint on the neck-posts to attach the arms correctly, the machines in China use a more sophisticated modern camera to detect torso print, and they don't need the paint on the neck-post. This is why minifigures made in China (collectible minifigures, magnet and keychain minifigs, etc.) do not have the paint mark on the neck.