Minecraft 1.5.2, Minecraft 1.6.4, Minecraft 13w42b, Minecraft 13w43a, Minecraft 1.7, Minecraft 1.7.1
Windows 8 64-bit, Java version "1.7.0_40"
Java(TM) SE Runtime Environment (build 1.7.0_40-b43)
Java HotSpot(TM) 64-Bit Server VM (build 24.0-b56, mixed mode)
For a long time now, the sign hitbox has been different from the text that fits on the sign.
Obviously the sign's hitbox is one block. However, the model is bigger than one block and thus gets overlapped when a block is placed on top of it.
This causes the first line of the sign to be unreadable. You might not consider this a bug, but it is definitely not a feature either.
Examples given in the attachments.