This bug has been filled here: https://bugs.launchpad.net/ubuntu/+source/nautilus/+bug/117795

"I found out some inconsistent behaviour when using search-as-you-type in nautilus file browser. I use Feisty, but the same applies to Edgy IIRC.

Steps to reproduce:
1) Go to some folder in the file browser
2) Type the first few letters of a filename (search box should pop up in the lower right corner)

Then do one of the following:
3a) Press enter
3b) Double-click on the selected file

What should happen:
The file is opened.

What happens:
3a) This works correctly.
3b) This doesn't. When double-clicking, only the search field goes away. After a second dbl-click, the file is opened.

Additionally: If you dbl-click (after searching) on the filename that was selected BEFORE the search - regardless what is selected during or after search - it will be opened directly on the first dbl-click

Steps to reproduce:
1) Open some folder in nautilus
2) Select a filename with a single click
3) Type some letters to start searching, but search for another file, not the one you selected
4a) Double-click on the file selected in 2)
4b) Double-click on the file selected after searching in 3)

What happens:
4a) The file is opened
4b) The file is not opened. Only a second dbl-click opens the file.
"

This is reproducible in trunk