When playing singleplayer or multiplayer Minecraft on a mac with no mod software installed, I was unable to type the maximum amount of characters on signs. The amount I was able to type varied from 10-15 characters on each line.

To reproduce this bug, run minecraft on a mac with all mods (OptiFine, etc) disabled and place a sign on a singleplayer or multiplayer world, and try to type as much as you can per line.