Rails can be duplicated when they are pushed downwards (or sideways) by a piston.

In the setup pictured, when the rail on a block is pushed downwards by the top piston and block, the rail is remains placed on the lower block, but also drops an extra rail, allowing it to be infinitely duplicated.

Also affects other types of rail also, though these break when pushed back up.

This issue was brought to my attention by means of this video: https://youtu.be/J0OeQ--oSxU