Sunset: The site plucked out a photo of San Francisco at dusk (which may not count, since I had "sunset" in the caption), and another of the sun setting over rural Maryland, but missed a panorama of a Seattle sunset. "Sunrise," however, found a shot of the setting sun in Austin that the prior search had overlooked.
Moon: Amazingly enough, it nailed two photos of Earth's satellite taken from my front porch (and a third that had a much smaller moon visible but used that keyword in its caption).
Sky: This pulled in multiple shots of the view above during the day and night - and one of a Windows-install error message that, with its blue background and reflected glare from an overhead light, does look uncannily like the sky.
BLOG: Handwrite Your Gmails
Windows: Google+ found three photos of windows and sliding-glass doors, then got confused by pictures of glass tiles and one of a framed photo. It did not, however, find that Windows error message.
Airplane: This was a total failure, missing all of the pictures I've taken of planes and instead only locating two iOS screengrabs that highlighted the airplane-mode switch. And yet "aircraft" quickly found four photos of various commercial jets.