For some reason, when on the fourth line of a sign, I am no longer able to exit it's interface. The X button scrolls up out of the screen's boundaries, making it unable to be pressed. At this point the only thing I can do is relaunch the game. I brung down the keyboard, but that only made it go higher off the screen. I disabled my texture pack and relaunched the game, still happened. Perhaps it's my device?
To reproduce:
1. Place Sign
2. Skip to the fourth line.