Glass panes have the connected texture as they would normally have with full blocks, however they are doing this connected texture if signs or levers are next to them.

I'm assuming this is a bug because it seems pretty odd to have as a feature. I know mojang changed how fences connect to stairs, but I don't think this was intentional.

Also maybe I'm wrong and they have always done that.... then woops, sorry!
Here's a screenie