When a piston pushes a rail into a space without a block below it, and moving the rail would otherwise cause another rail next to it to change position, the game crashes. See picture above.
Crash report: https://gist.github.com/JBYoshi/e6001403c01b4d09a32a3abc28f4f4c3
Originally reported at https://github.com/SpongePowered/SpongeForge/issues/2920. However, this issue still occurs on a blank Forge server.