If the Unity Dash is hidden (using app maximized) I make a selection in a GTK app and I drag-n-drop it, the Unity Dash shows up. I'm not sure it's a feature or a bug, but sometimes it stays there and doesn't wanna disappear. The only way I could manage to hide it was to move a selection behind it and back.
It behaved so when I tried it with Chrome, Firefox and Nautilus, and it didn't with MySQL Workbench and Netbeans IDE.
EDIT: Actually I managed to stick the dash also with Netbeans, had to move a tab.
EDIT2: Filed the bug: https://bugs.launchpad.net/ubuntu/+source/unity/+bug/777203