Reported here:
https://bugs.launchpad.net/ubuntu/+source/gtk+2.0/+bug/509086

Quoting the original description:
I'm experiencing a strange behaviour in the Open/Save dialog (gtkFileChooser). When I select a folder (with a single click) the subsequent double-click doesn't open this item. I have to perform once again a single-click and then a double-click opens the folder.

Steps to reproduce:
1. Open any GTK application which has an Open/Save file dialog (for example gEdit).
2. In the menu select the Open (or Save) item to bring up the file chooser.
3. Select a folder so it gets highlighted (single-click).
4. Wait a moment.
5. Now try to open the selected or any other folder with double-click, it doesn't works!

To make it work you have to perform another single-click and then the subsequent double-clicks will work.

It looks like the clicks are counted in the dialog and if the double-click is composed of an even-odd (2.-3. or 6.-7. ...) click of the sequence of clicks it doesn't respond. When the double-click is composed of an odd-even (1.-2. or 5.-6. ...) click it works.

Ubuntu lucid (development branch)
libgtk2.0-0: 2.19.3-1ubuntu1