Summary:
When you place signs on fences, nether fence, cobblestone wall, mossy cobblestone wall, glass pane, or iron bars, it causes them to float where the fence/other should usually connect. I understand that the fence acts like a normal block, but you could still allow the fence to connect to the sign. I'm not sure if you would count this as a bug though, since it wouldn't happen if it wasn't intended. This has gone unfixed since Alpha 1.0.17 when fences were added.

What I expected to happen was:
The fence to connect properly to the sign.

Describe what you thought should happen here:
The fence should have properly connected to the sign.

What actually happened was:
The sign floated without the fence connecting to it.

Steps to Reproduce:
1. Put a fence (Or another block of that type) block down
2. Put a sign on the side of it.
3. Bug reproduced.