Bug discovered when using Mapcrafter https://github.com/m0r13/mapcrafter/issues/81 for Minecraft in Multiplayer, server 1.7.5, clients 1.7.4 (In release notes to 1.7.9 no mention of Signs so assuming 1.7.9 is affected too).

Minecraft is adding invisible character to sign text when placed on Mac OS X (pressing Arrow Down, to edit next line).

In game everything is OK. Signs created playing Minecraft on Windows are fine, but those placed on Mac OS X have this character inserted.

Inserted character is 0xf701. Apparently FN+Arrow https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSEvent_Class/Reference/Reference.html ( NSDownArrowFunctionKey = 0xF701 )

Mapcrafter is accessing sign text parsing world data.