What I expected to happen was...:
The text entered gets cut off if it's to long.
What actually happened was...:
By entering some special characters (tested with ~), the maximum amount of text that fits into the sign is larger than the sign. ~ is the only character I know that can be used in vanilla to cause this bug. In custom servers or with a modded client, utf8 blocks can also cause it.

Steps to Reproduce:
1. Place a text and write the maximum amount of ~ inside.
2. Parts of the text appear out of the sign