When editing a sign, using down arrow key to move to the next line inserts an unprintable character is at the beginning of the line. This isn't a huge deal with Minecraft, as players won't see the unprintable character, but it breaks Craftbukkit plugins that use signs.
Steps to reproduce:
1. Log into a Minecraft server from a Mac Minecraft client
2. Place a sign with the following text, using down arrow to move to the next line:
Text on sign should appear as typed.
Text on sign has unprintable characters at the beginning of lines 2, 3 and 4.
Also tested this on Windows 7 64 bit and Ubuntu 13.10 64 bit and did not get any unprintable characters. This issue appears to have started with the 1.7.x series.
I used MCEdit to view the text of each sign, because Minecraft doesn't display anything for unprintable characters.
Attached MCEdit screenshots to show signs placed with a Windows client (sign on the left) and a Mac client (sign on the right).