What was expected to happen:
When the signs only support is broken, the sign should break, too.

What did happen:
The sign stayed suspended in mid air.

Steps to recreate this:
1. Place a sign on any block that signs can be placed on.
2. Destroy the block that the sign is placed on.
3. Observe that the sign is floating without any blocks around it.

I know there are a lot of reports of floating signs, but most of them are actually features in the game (signs attached to fences and other signs seem to be floating), but here the sign floats with nothing attached to it at all.
I took a few pictures at different angles just to prove that there are no invisible blocks and that the signs hit box is visible from the back and bottom. If the sign were attached to a block, the hit box would not be visible from those angles.
As you can see, I did this in mid air, so there's no way there are any blocks anywhere near the sign.